'Powder Blue' sugar paste

Wilton do a gel colouring in cornflower blue, probably the best route to powder blue icing. Apparently it's difficult to get that particular shade outside of the US but this place in the UK has it http://www.cake-stuff.com/edibles-c1/icing-colouring-c15/paste-gel-icing-colouring-c19/wilton-cornflower-blue-paste-gel-icing-food-colouring-28g-usa-p1318, have never ordered from them though.

Edit: when I've needed to mix a specific colour of icing in the past I've looked up artist's paint mixing guidelines and used that advice, for example to mix cornflower blue they suggest mixing white with ultramarine (a warm shaded blue, i.e. no yellow/green tones) and a drop of black to create the slightly grey (powdery) tone - the same principle applies, handy when it's hard to buy the colour of sugarpaste you want.