Jay,
Good questions...
White balance is the camera or correction adjusting for artificial light. For instance, if I am shooting an image and I have the white balance of the camera set for indoor tungsten light but I am outside, the image will look very blue. The White balance of the camera is thinking that the light hitting the imager is tungsten about 3200K where as the daylight actually hitting it is 5500K or so...
The image would have a heavy blue cast; so part of the answer is yes color balance and color cast can be related. When we are "generally" talking about color cast we are talking about a color leaking into the image that does not belong. In the image posted here
http://www.curvemeister.com/forum/index.php?topic=2565.0 there is a strong green cast over the face of the child. It also bleeds down into other parts of the image but the face and skin is where we really hate to see it.
The trouble with this condition is that setting a neutral is going to get you close only if the neutral is experiencing the same color cast. If the neutral is not in the same light as the subject you could have a proper neutral but still have the cast on the subject. This is what happens to the child in the image above. If you fix the child the background goes "BOOM" if you fix the background the child is green...this is where masks and selective corrections come in.
Lastly if the entire image has a cast then setting a neutral should fix the major color issues in the image. the real trouble is getting a good neutral as you yourself have experienced. We make certain assumptions to get the neutral. It relies heavily on perception and sample points. The more you rely on the numbers sometimes the harder it gets. In the Abe video you saw that the neutral can be "shopped" around to find sweet spot that makes sense. If there is not clear neutral, that is the track I follow most often; and it is why I try to teach that shopping method to all of you. Your eyes can really tell you a lot if you trust them.
LAB: Lightness is the channel that makes the most sense. When I first hit LAB I was astounded at what I thought was the complexity of the whole thing...then I hit an image that Mike used to use to teach LAB color cast correction. It required that you be able to split the B channel in two with your correction. It was an eye opener. Here's how I now look at A and B...
In RGB you have three colors for correction, obvious and seemingly simple...but you cannot adjust RGB without messing with the color, all of it...you can select the R channel but you effect all of the R numbers when you make a move in there up to a certain point. The further the difference in brightness the less the effect, if you have two colors fairly close in brightness you can really mess with the color by adjusting only 1 channel.
In LAB you can isolate the color you want without messing all the others up. The A channel has two colors Magenta and Green...but at the very center of the grid is neutral L(x)(a0)(b0) this is the best part of LAB it allows you to separate the Magenta correction from the Green one. When you set a neutral in LAB you tell the image that the point I have chosen is now the center of the the A and B curves.
This is why you have only 1 neutral in LAB. If you place control points on the Magenta side of the grid...three equally spaced points should do, then crank on the other end of the curve you should be able to totally wack the greens out of this world without touching the magenta or reds. Week 4 has 2 images where you get to play with this and It will help if you understand what is happening first.
Now, what if I want to increase the reds and kill off a blue cast in the shadows? I would pin down the green end of the A curve and adjust the reds to my hearts delight...then pin down the yellow side of the B channel and kill off only the blue in the shadows.
As you look at LAB remember that the closer to vertical the curve is, the more saturated the colors are. Most of the action is near the center of the grid. There are techniques we are going to cover that increase the color variation of green vegetation because we isolate it in the A channel and then steepen the curve only in that area.
Lastly, LAB is capable of creating colors that cannot be produced by RGB. A extremely bright yellow for instance in LAB would be 80L (-5)A and 80B that would be = 255,255,255 in RGB because it cannot handle the colors, they are out of Gamut and RGB would try to come as close as possible but be unable to produce it. When you convert from LAB back to RGB it would assign pure white to the very bright yellow from LAB.
A extremely good book on LAB is "Photoshop LAB Color: The Canyon Conundrum and Other Adventures in the Most Powerful Color Space" By Dan Margulis. I have read it now twice and I am re-reading it again because every time I do I find something new to add to the class and my own personal knowledge. Dan can be very direct and sometimes confusing but if you relax, open your mind a bit and only try to get 1/4 of what is there, you will learn a ton.
Keep asking...I'll keep trying to give you an answer that makes sense...
PLEASE MAKE TIME for the conference call on near the end of the class. I think you will get a real benefit from it. Most of your questions are easily demonstrated and when you see that you will have totally different questions, but a much greater understanding. I hope... ;D
Greg