I don’t think matching the color of A&A pieces would violate any copyright or trademark, at least not by itself. I think FMG avoided that by simply calling them “Combat Units” and not “Axis & Allies Combat Units”. Also, FMG is producing different units from WOTC (eg. Folgore fighter instead of brown Me 109) and even the infantry units are in different poses. So even if they were the exact same color, FMG would not be violating copyright because the pieces will still be different. Plus, they state that the pieces can be used for Axis & Allies and other WW2 games so that saves them from copyright issues as well.
I think even minor differences can protect you from copyright infringement. Take the new German Artillery piece from FMG. It’s an 88mm with a blast shield. WOTC’s German artillery is an 88mm but it doesn’t have a blast shield so technically it is a different piece. On a personal note, I think the new FMG German artillery looks really cool. Can’t wait for more German sculpts.