my buttercream always seems to split from the colour. you can literally see it. cant understand why

The problem with colouring buttercream is that most colours are water based and buttercream is of course mostly fat - oil and water don't mix.

Make sure you use paste or powder colour and not liquid colour. Dark colours are often problematic. When making black start from chocolate buttercream.

Darker colors are more likely to do that because of the amount of food coloring that you have to put in them. Also agree with the oil-based food coloring option, especially if you're using meringue buttercreams.