After Effects: how to create a color via expressions
From what I'm reading, it looks like it may just be [R,G,B] where each value is a floating point number between 0 and 1.  I'm looking in the Adobe After Effects Scripting Guide.  Every method I can find that accepts a color value simply uses an array like that.

Update: While certain scripting methods take a 3 parameter RGB value, other expressions require RGBA or HSLA.  In these cases, there will be a forth parameter for Alpha (ie, transparency) where 1 is fully opaque (a good default value) and 0 is fully transparent.  If you get an error with one format, I suggest trying the other and if you get unexpected values, check if the input expects HSL or RGB.