So lets say I want to make a rainbow effect in a java applet.

Color rainbow = new Color(red,green,blue);

Where did the vars red green, blue come from? Lets just say they're declared random integers. That's not the important part right now.

Ok so now that I have the color 'rainbow' outside of a while/for loop, how would I change this already made color? I've tried looking for this forever but I cant find it! All I can find is Color <name> = new Color(r,g,b); nothing about changing existing colors. I can't have a color for every single value out there from 1-255 * 3 that'd be at least over 9000 color declarations.

I'm not a Java programmer, but I'll try to help you anyway.

If I understand correctly, you want to change the color of "Color rainbow"?
Well, I think you just can issue
Color rainbow = new Color(red,green,blue);again, just with other values for red,green and blue.

Edit: if Color is a type, I think you can leave it away, as the var is already declared.

A quick google search on the subject turned up this webpage.
So the code displayed that site that would generate a random 24-bit color would look like
Random r = new Random();
Color  c = new Color(r.nextInt(256), r.nextInt(256), r.nextInt(256));