SC 2154 WW1 Veterans

I have three copies of this stamp. Two are on regular white paper. The one that is nicest is on fairly bright yellow paper, however. I see no mention of this in Scott and the pic shows white paper.

It doesn't look as though it is dye off an envelope. It is yellow uniformly across the whole stamp and equally on the front as well as the back.

Can someone tell me what's going on -- little as they are worth, I don't want to keep a hopelessly flawed example just because the postmark is lighter.

Thanks!