Confusion

So after painting my first trygon I noticed that the blue carapace seemed off. Figuring it was the light I packed up for the day. However the next day at the den it was very apparent that the ultramarines blue I was using for the carapace had turned to a pastel color. Any ideas what would cause this?

Were you painting over a white highlight? Did you shake the pot before using it? If its over a white hightlight, there you go. If you didn't shake it , it may have been the acrylic polymer seperating from the pigment, which would create a thinned effect.