There is no "Color-coding convention" per-say.. However, you'll notice that Agilent (HP), R&S, NI (and others) use typically the same color schemes.
I followed their trend and select a color scheme by functionality. Use one color for RF, another for digital IO, etc. That way, they are easier to group and create libraries. But as far as an established rule on colors, I am not aware of anything "formal". Let the artist in you come out in your icons... ;o) -JLV-
