Ike
First, make sure to convert your grayscale image to a color mode if
needed - RGB, or CMYK.
The easiest way to color a grayscale image is to use the "Color"
blending mode. You can use any of the paint tools in Color mode, and
paint right over the grayscale pic. The colors will take on the
brightness of the grays underneath.
Better yet, make a new blank layer above your gray pic, put the layer
in Color mode ( in the Layers palette), and then you can paint (in
Normal mode) on the new layer. You can work faster because erasing
color is as easy as applying it.
Another method that gives you more control is to make a selection of
something you want to make a particular color, then create an
Adjustment layer (Ctrl-click the "Create new layer" button in the
Layers palette) using Hue/Saturation. Enable the "Colorize" option in
the Hue/Sat dialog, and adjust the sliders to get the color you want.
You can go back and change the settings at any time so you're not
committed. You can make a separate Adjustment layer for each different
area you color.
TIP: Choose the color you want before using Hue/Sat. When you enable
"Colorize", that foreground color will be used.
Notice that when you make an Adjustment layer with a selection active,
you make a mask for it. Look at the thumbnail in the Layers palette
for the Adjustment layer. See that white indicates where the
adjustment is applied, and black where it's not. You can paint, fill,
filter, use a gradient, or anything you need to change the mask. Gray
values on the mask make a partial mask, meaning you can apply the
adjustment partially wherever you want. For example, say you had a
checkered tablecloth that you'd colorized with red. You could choose
one of the values from the grayscale image (say, the lighter squares
of the pattern), then fill the mask with gray to lessen the
adjustment, making the lighter squares less saturated than the darker
ones.
Selecting areas before colorizing them with an Adjustment layer is
more work up front, but it allows you a lot of flexibility. Making a
colorization less flat looking is easier when you can alter the mask
that applies the adjustment. Making selections based on highlights or
shadows, for example, allows you to lower saturation in the
highlights, or make a cooler hue in the shadows. You can add noise or
mottling (Clouds filter) to a mask so that colors are not so uniform.
You can darken/lighten areas to create more contrast in the grayscale
pic. You can paint on the mask to remove color from spots (eyes,
specular highlights), soften edges, etc...
Ross