I haven’t used these colours in Rose’s buttercreams but have in non-cooked beaten buttercream and marshmallow fondant icings….here is an excerpt of a previous post I made on red and black fondant colouring which may assist. I’m starting another Thomas the Train cake this evening and the powdered colours are what I will choose, can’t beat them for depth in dark colours IMHO and you only need the a few grains….less than 1/4 teaspoon of black powdered colour did a whole batch of fondant.
FROM post Oct 9: As Hector says, you do need more colouring to make these shades and this will throw the liquid ratio out enough to mess things up! Also the shortening ratio although small is critical to the elasticity and smoothness of the final product in a fondant.
I have used both Wilton paste and Corella powder colours and have found the powder colours perform best. You do need to dissolve the powder colours in a few drops of warm water (e.g no more than a teaspoon) otherwise they will appear grainy in your fondant, but the depth of colour is far superior to paste IMHO!
Also, I never start from white when making black and red. For red I add the tinest amount of black prior to adding the red which gives better depth.
When making black (depending on the amount I require) during initial mixing I add a tiny bit of blue then black and the hand knead in more black.
I also find that I use about 1 teaspoon extra shortening to grease my hands when kneading (we have a different kind here in Aus to that which Peggy specifies) and occasionally about 1 tablespoon extra of icing (confectioner?s) sugar if it gets sticky to keep the glossiness and texture.
I haven?t tried it but I have heard that dissolving pure cocoa powder (e.g. not drinking chocolate) in a little warm water and adding extra 1 teaspoon shortening in the intial mix gives a good brown.
Hope this helps!
Another thought (and others here know much more about it than me) - had you considered spray on colour?