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. :)
I use EDIBLE Sugarflair colour pastes to add colour… With the blue, add a touch of baking soda to hold the colour :)
I can’t add links from my iPad but if you google “Sugarflair colour paste” you should find it… A lot of suppliers sell it and post outside the UK. .