Concerning going from grayscale to color...
Traditional oil painting has a step called "under-painting" or "dead layer" where the painter creates the values of the piece -- a grayscale version of the final painting. Over top of that, the painter then applies a small amount of color (think of it like a glaze) so that the dark/light value of the under-painting shows through.
In photoshop this can be done by scanning in a grayscale image, then painting over the gray with colors at various opacity.
Read up more on the subject by searching for the terms underpaintingand dead layer