I use Americolor Super Red. I have never used gumpaste, so I am not sure how much you need. And something else: when I make red frosting, I always mix it 2-4 days ahead so it has a chance to saturate well. And if it needs more I can add more, as time goes by. I have discovered that if I wait till the day I need it, I have to use so much red color that the cake will ooze color if you have cake leftovers for more than a couple of days. I hope this helps. 
If gumpaste is like flowerpaste and dries really hard. This will work (I have never use gumpaste and not really sure what it is)
I use a small amount of vodka and colour it to what I want then coat the flowers and the spin them upside down to get the excess colour off. I did this with the flowers on the wedding cake. The flowers were also a deep red. Also if on the first go the colour is not right wait until they are touch dry darken the colour and try again. It is also really messy but fun!
http://www.earlenescakes.com/store/colors.html
Scroll down to the bottom of the page. Earlene Moore used this red red paste to get some really really red roses on one of her cakes.
These roses took a whole jar plus a little of another jar of Super Red coloring from Americolor, I think... I put the color in when I was making the gumpaste. I make Nick Lodge's recipe and when I make the royal icing I add the color. It tends to be a bit stickier when I do this so I add more powdered sugar.
Thats nice RED.... That is what I want ...deep red.... 
Your flower is from gumpaste? and the cake is coverd with buttercream. is it? Isn't too heavy for the roses to sit on the buttercream? What buttercream recipe do you use? sorry for the many questions as my buttercream recipe never crust like that....
no. the americolor has no taste!!!! By the way......why not get your gumpaste as read as you can and then just use the red petal dusts??? If you're looking for a really dark crimson red, adding some black to it (just a touch) really helps to darken it up
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