You may already know that there were several permutations of hand guns that eventually became the P-38. As to your question, the original design recognized as the P-38 was produced in 1939 and 1940. These were all equipped with black handgrips but were easily identified by serial numbers beginning in "0" (example 01234). These are very rare. The first colored hand grips appeared on the "AC" series in about 1941. The grips are somewhat reddish brown but the shade can vary from lighter to near-blackish brown. Identified by several design changes, but with serial numbers that have AC followed by the production year (e.g. AC-43j over or before the SN).
The serial numbers can be confusing in the earlier (pre-1944) productions since they began again at "1" at several junctures of design change (although, the number is followed by a small case letter for various years as shown above)... Good luck!