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How do I change the color of an actual JButton in Java? Not the background and not the text

button[currRow][currCol].setBackground(Color.RED);
button[currRow][currCol].setContentAreaFilled(true);
button[currRow][currCol].setOpaque(true);
That's what I have right now for my connect four game to denote a red player's move.
At the moment, it only colors the background and if I change my code to button[currRow][currCol].setForeground(Color.RED) then the whole thing just appears to not change. How would I fix this?

This is not easily achievable. The problem is that the pluggable look and feel paints the button content, and it does so in whatever way it sees fit. For instance, some L&F might paint a gradient which does not use the background color.
I suggest for a case such as yours to use a custom image (JButton.setIcon()) and no content area (JButton.setContentAreaFilled(false)).
Alternatively, you could create a custom component which draws the element itself, overriding JComponent.paintComponent().

Java how to draw and fill a Polygon which has holes

I am currently trying to draw and fill a Polygon which has a hole in it in Java. Normally this would not be a big problem, since I would draw the exterior ring and then draw the interior ring with the color of the background.
But the problem is, that the polygon is displayed above a image which should be "seen" through the hole.
I am writing the code in Java and am using JTS Topology Suite for my geometry data.
This is my current code, which just paints the border and fills the polygon with a color.
private void drawPolygon(com.vividsolutions.jts.geom.Polygon gpoly, Color color, Graphics2D g2d){
java.awt.Polygon poly = (java.awt.Polygon)gpoly;
for(Coordinate co : gpoly.getExteriorRing().getCoordinates() {
poly.addPoint(co.x, co.y);
}
g2d.setColor(col);
g2d.fill(poly);
g2d.setColor(Color.BLACK);
g2d.draw(poly);
}
Sadly java.awt.Polygon does not support Polygons with holes.

Use the Polygon as the basis for an Area (e.g. called polygonShape).
Create an Ellipse2D for the 'hole', then establish an Area for it (ellipseShape).
Use Area.subtract(Area) something like:
Area polygonWithHole = polygonShape.subtract(ellipseShape);

There are some ways to draw shapes or areas that are more complex than a simple polygon (another answer already mentioned Area).
Besides those, you could try to tessellate your final polygon. There are lots of algorithms to do this. For more complex shapes, the algorithms get a bit more complex as well. Basically, you're dividing your final shape into little polygons (usually triangles, but it also can be something else) and then draw those polygons.
You can take a look at your possibilities by searching for "Tessellation Algorithm", there are also some already implemented libraries for Java.

You can use java.awt.geom.Path2D to render a "compound shape" with a hole in it:
If you have java.awt.Shape objects defining the outside & inside edges of the shape, use append(shape, false) to add each shape.
If you have a set of path points for the outside edge and a set of path points for the inside edge, use lineTo() to add the first set of points - creating a closed loop by either ending with the same point you started with, or calling closePath() to automatically close the loop. Then use moveTo() to create a break before adding the inner set of points via more lineTo() calls.
In either case, you must create the path passing Path.WIND_NON_ZERO to the constructor - otherwise the hole won't be left unfilled.
See How to create shape with a hole? for a longer code example.

You could fill the polygon first, and then draw the holes on top, giving the illusion that it filled everything but the holes.

How can I get a certain RGB value from a canvas painted with LinearGradientPaint?

I have used the LinearGradientPaint class to draw a rectangle dynamically filled with user-defined colors. This happens by overriding the paintComponent method.
Here is how it looks like:
.
You can see the small thumbs/ handle on top. The user can move these around, delete them, and add new ones. They can also change the color. As a result the user can completely customize how the gradient looks like. This works just fine so far, no issue.
What I need now, and I tried searching for this info, is to get RGB-values anywhere on this gradient.
I only know the x-amount of colors that LinearGradientPaint uses to generate the gradient. I know at what point (fraction) these colors are located (the number below the gradient box, corresponding with the 'thumbs' on top.
Is there anyway to get the colors in between the RGB-values which are used to generate the gradient? In my example above I mean the darkened red or green areas.

If this really is a linear gradient between new Color(r1,g1,b1) and new Color(r2,g2,b2), then the colour at x, where 0 <= x <= 1 is
new Color((int)(r1*(1-x)+r2*x),(int)(g1*(1-x)+g2*x),(int)(b1*(1-x)+b2*x));
Of course, I have no idea whether this is the formula that is actually used inside LinearGradientPaint - but it ought to be something equivalent to this.

A way you might be able to do this, is to create a 1 pixel high BufferedImage of the same width of your component, render the gradient to it and simple use something like BufferedImage#getRGB.
This will return a int packed color value, which you can then use Color(int) to return a Color object, which makes it easier to extract the color components of the pixel.
Of course, this would all be easier if you used the BufferedImage as your primary output as well, then you would only have to create it once and because you'd be updating the image so it could be rendered to the screen, it would also be up-to-date

Detect lines and its curve hand drawn by Graphics2d in a java draw application

I'm making a music application that enables you to freedraw lines from specified colors and then a trackbar would pass on top of it and should detect the curves and color to output sound and modified pitch etc depending on the curves.
What I'm looking for is help on detecting the lines and curves. They are drawn in a Label as a BUfferedImage. The following is a screenshot:
The Black line I drew to represent the trackbar, but infact it would be a Rectangle drawn by drawRect or it could be a imagepanel, depends on the way you detect lines.
My question:
How could I detect the yellow, green, etc lines and its curve and handle that data? If I get that data I could easily modify gain and pitch for each assign sound for each color. Thanks!

You want to store the coordinates of the user's drawing motions at the very least. You could store these as points or convert them into representations of line segments and/or curves depending on your requirements. Of course, store the stroke, color and anything you want as part of this model.
To accomplish this, I think the path of least resistance would probably have you utilizing java.awt.Shape and everything in java.awt.geom. Specifically, I think you would want to represent each user-drawn element as one or more Path2D and/or Area objects in response to user drawing motions. Then, have your GUI render these models on your custom Component.
Path2D would store the movements as lines and curves accessible through a PathIterator. Given a trackbar represented by a Rectangle2D, you can use the various methods available for comparing and combining geometries--namely the Shape interface's intersects(Rectangle2D) here. If a geometry model intersects your trackbar, you could then iterate through the path components until you find the actual sub-segment that is intersecting. This gets you the local slope.

Mouse pointer detection over a Path2D

I have constructed a Path2D that represents an unclosed shape consisting of straight lines:
I want to be able to detect when the mouse is clicked and the mouse pointer is near to (within a few pixels of) the path. Using the contains method does not work because the algorithm treats the unclosed shape as implicitly closed (i.e. by drawing a straight line between the start and end points).
Does anyone know of another mechanism for achieving this?

Create a BasicStroke (the width controls your pixel-distance-tolerance)
Don't draw with it, only use its createStrokedShape method to create a second shape from your shape. This second shape describes the outline of the shape that would be filled if you would draw your first shape with the BasicStroke.
Use the contains method of this second shape
From Stroke.createStrokedShape API documentation:
Returns an outline Shape which encloses the area that should be
painted when the Shape is stroked according to the rules defined by
the object implementing the Stroke interface.