Looking at GraphicsUtilities.decodeColor(String) it says that any of the standard Java Color values is supported. But, trying to use "darkgray" or "lightgray" doesn't work.
The underlying reason is that the lookup is done using the lowercase value of the input string and trying to find the corresponding field name in the Color class. But these two static fields are named "darkGray" and "lightGray" (mixed case) and so the lookup doesn't work.
Need to special case these two values. Wouldn't hurt to support the British spellings ("darkgrey" and "lightgrey") either.