PSL has exceptionally rich set of commands for controlling colors.
In addition to an interactive color definition capability available in the PSL development GUI, the following controls are available programmatically.
Again, wherever the properties for a color or an image can be set, they can be found.

Set a Color by name – dozens are predefined and users can create as many named colors as desired
Set the Color Intensity – from vibrant to washed out
Set the transparency mode – pick from 16 possibilities
Set a range of color as transparent
Set the Opacity of one image relative to another
Define new named colors by r,g,b content
Define new named colors by c,m,y,k content
Define new named colors by hue, saturation, luminosity content
Define a pattern by giving its pathname and scale factors in x and y for the tiles
Define a new color lighter or darker than the named color by fractions of its components - useful for transparency