AI've recently started using a new product called sugar & crumbs flavoured icing sugar... I've been adding sugarflair colours to them and on 2 occasions now they seem to separate apart. You can still sort of see the white powder, like it hasn't been mixed properly. But it has. Anybody have any ideas on what this could be or how to solve it without losing flavour?

ASorry, never heard of that product, but have you tried using a different coloring product? F F.e. Gel instead of powder or vise versa?

Well.............I'll weigh in here.

UK icing sugar is very pure, without cornflour or fillers, and I assume that's what you are accustomed to using.

The flavored icing sugar has ;lots of "stuff" added to it---flavors, starches, "carriers", and anti-caking agent [likely cornflour, but it could be something else, too]. 

I doubt that those things are playing nicely with your recipe and your colors.  Some of them may not dissolve at the same rates, or take color at the same rates, resulting in those white spots.

I flavor my buttercream all of the time using extracts, compounds, preserves, etc., and with some of those, I may have to change the amount of liquid I add to the recipe, but they don't seem to affect coloring. 

I have to say, that I feel the "flavored icing sugar" is wayyyyyy more expensive than doing it yourself--for me, it would be more than 6 TIMES what I pay for plain powdered sugar!!!!!!!!!!!!!