red : *0.8 ("red" is darkened by factor 0.8)
#ffeecc : +16 ("ffeecc" is going to #fffedc because 16 is added)
The color can be given as HTML-color or as a comma-separated list of RGB-values (integers).
You can add an extra parameter that will modify the color mathematically:
[colordef] : [modifier]
where modifier can be an integer which is added/subtracted to the three RGB-channels or a floating point with an "*" before, which will then multiply the values with that factor.