Yes, I play with that too. ( muscles) And yes, its good fun.. luv learning it through.

When the skin is white, the more solid the white color, the more influence that joint has on that part of the skin.. so... if say, the upper arm is all solid white, and you have the elbow joint selected, that joint has full control on that area, if you want some of the upper arm to be influenced by that joint, but the upper portion of the upper arm to be influenced some by the shoulder joint, you can start fading the white to varying grey, to black as you move up the arm by painting the white away, solid white nearer the elbow, fading to black as you move up the arm....then select the shoulder joint, and doing the same.. Just remember, the more solid the color, the stronger the influence from the joint selected on that part of the mesh...if the mesh is deforming wrong because it has too much influence from one joint, paint it down some, ( greying it ) A good example is a shoulder, it is influenced by the arm and the torso, so when you move it from the T position, to arms at its side, it could "fold" instead of looking natural. By painting how much influence the mesh recieves from that joint on the torso, and how much it influences the arm and shoulder, you can get a more natural look when its in the down position. I think you know what I'm saying here, Stark black is no influence, stark white is full influence. The greyer it gets, the less the influence on the skin at that place. Sorry for the long explanations.... Let me know if your still having problems getting the hang of it, I'll try to search for the online tuts I saw awhile back for it.. it wasnt very detailed, but did a better job of explaining and visualizing it than I can do.
Have fun!