re: How can I add a deep royal blue color to a gum paste or fondant flower?

I recommend pre-colouring the gumpaste with food colour paste or gel, in the UK Sugarflair is a fab one to go for and they do a lovely Royal Blue in the range, you can then add depth to the flowers by adding some blossom dust on top, this gives the flowers a really lovely even finish and allows you to deepen the colour to add depth in certain areas. :)