We will try to keep a list of the best matching paint colors here for you.
GRD engines: The closest color that I have found so far for the GRD is made for Ace Hardware.
It is in their "Rust Stop" line. Part number: 17138 and it is called "International Green".
BLD engines: The other day, someone had mentioned that "Dupli-Color", pn. DE 1608 G.M. BLUE would match the original engine color pretty closely. I am looking into this further and will post the info.
If you know what paint colors/mfg./etc are real close for other trucks, please post the information in a comment below.