And I just checked The second definition of "primrose yellow" ("primrose" itself not being listed as an adjective) was light to moderate yellow:

"Main Entry: primrose yellow
Function: noun
Date: 1882
1 : a light to moderate greenish yellow
2 : a light to moderate yellow "

Edit Addition: And I just checked my AHD (1992) and primrose is not listed as an adjective at all. I, for one, am going to start using primrose as an adjective all the time until somebody comes up with another color adjective that means light yellow.