If you created the other diff before, and the new one has combo colors that you just added, the combo colors are not universal. You'll have to go into the .osu file of the first diff and scroll down between [TimingPoints] and [HitObjects] and copy the [Colours] there and paste it into the new difficulty's .osu in the same spot and save.
(This is assuming when you said 'map' you mean another difficulty soz)