Fie on Photoshop: Krita, the Real Photoshop Killer - page 3
The LMS cone color space aims to map colors to the physiology of the human eye, and its sensitivity at different light wavelengths. You remember from grade school (and this is the simplified grade-school explanation) how our retinas have rods for perceiving brightness and shapes, and cones for color. There are three types of cones: long wave, medium wave, and short wave, hence the name.
YCbCr is a special family of color spaces used in digital component video. Y is luma, or brightness. Cb and Cr refer to the chroma, or color signals. This was originally invented so that the same TV signals could be used by both black and white sets and color sets. It also allows colors to be re-sampled and compressed, and to balance brightness and colors to accomodate how the human eye perceives them. YCbCr isn't an absolute color space in the way that RGB, CMYK, and the others are, but is a way to encode RGB. There aren't any reasons I can think of to use the YCbCr profile when you're creating or editing still images, because it uses lossy compression and a small color gamut. If you want to separate brightness and hue, use LAB.
Grayscale is, well, no colors. Just black to white and all the grays in between. But a whole lot of grays- at 16 bits, you get 65,536 values per channel, which gives you a lot more accuracy and subtlety than the 256 values at 8 bits.
The Watercolor simulation has a 14-color palette, with different degrees of wetness and paint strength. LMS and Watercolor are not used in any ICC color-matching profiles, so don't make yourself crazy trying to find some.
The worlds of color spaces and color management are vast and complex, and now you know more than the majority of Photoshop users.
You can read all about RGB, CMYK, and other digital image editing fundamentals in part 1
Dan Margulis' Books and Articles
The Krita developer blogs are essential reading: