Jeremy,
They have used the same colors since 2004. That is 7 years of consistency, and the vast majority of the current product in the market uses the same base colors.
All of the below use the same colors for each power. The inconsistency came about from 1999-2003, as WotC/AH was moving away from the 1984 edition colors and it took a while to standardize. Since then, the colors have remained consistent.
Axis & Allies 1942
Axis & Allies Europe 1940
Axis & Allies Pacific 1940
Axis & Allies Anniversary
Axis & Allies Battle of the Bulge
Axis & Allies Guadalcanal
I am a piece junkie, and I admit it - I have probably 10,000 Axis & Allies pieces in total. I have every color version that has come out, including lime green UK pieces. However, I realise that the colors have been stable, and will be stable, since 2004.