So sorry about leaving you in suspense there, I’d meant to post more but was in a hurry earlier.
Because of the texture and the high fat content, Mousseline just does not take strong colors well if you are using the regular paste/gel colors. You will end up using the entire jar or more and still end up with a mottled, pale looking red. Adding so much color will also alter the texture quite a bit. The Wilton-type paste colors work best with American buttercream (butter/shortening/powdered sugar). I find the No-Taste Red is very weak, use Red Red, if Wilton is all you can find, and do consider using the Wilton buttercream recipe. I know, I know, it is inferior to the Mousseline
If you can get it,try Americolor Super Red for gel color.
For Mousseline, a far better choice is the oil-based color made for chocolate. These are often called “Candy Colors”. They blend in with the buttery Mousseline much more easily. I have only used Americolor (Super Red is the one you want), but I think Ateco and Wilton may also make these.
Hope this helps