If you have Photoshop and are frustrated by the in-program color picker, here's something you may really, really like.
Anastasiy, over at anastasiy.com, created an awesome color picker panel for Photoshop CS3, CS4, and CS5 which acts just like the color wheel found in Corel Painter, plus it has a few extra neat features.
What I like best about it is that the panel stays open all the time. I don't know why Adobe hasn't understood yet the importance of this yet, but even their newest HUD falls short.
(I hate the HUD. It takes holding down two keys plus a stylus button to open it, and then I need to alternately press or not press the space bar with one of my remaining fingers to change colors.
It works with both RGB and CMYK modes, can resize to whatever size desired, and even minimize like other Photoshop panels. It has a lot of neat features packed into it.
I've taken to checking my Photoshop documents for value/contrast lately. I used to create a separate copy of the image and desaturate it, or use an adjustment layer, but wanted to see if there was something faster I could use.
I discovered the Proof settings in Photoshop include all sorts of color modes, including a bunch of black and white modes. You can find the proof setup menu here:
The nice thing about this is I can set up a custom mode to one of the black and white color modes. Then all I need to do is use the short cut keys to turn the preview on or off. Below is the mode I use to check values in my paintings.
PRACTICAL LIGHT AND COLOR
Lastly, I picked up a copy of Practical Light and Color, published by the Gnomon Workshop.
I thought I had a pretty good understanding of light and color before watching this, but it sure opened my eyes to things I never considered, let alone knew, before.