I need to make rose gold buttercream for a cake. Just wondering how to achieve this colour. Any ideas? 

The only thing I know to try is to use pink gel color, a tiny touch of black gel color, and a good helping of gold luster dust mixed in.  On the other hand, it always seems to dull the gold when the dust is mixed with something.  Perhaps it woud be better to frost the cake a soft dusty rose (pink with a touch of black), then, after it crusts, airbrush it with a glittery gold paint.  If you don't have an airbrush, Wilton makes a spray can (like a small hair spray) of edible gold that works beautifully.  If it is a big cake, it will probably take more than one can.