Lets take r as the example here.
On the first run of the loop, r is set to 0. Then it is set to 0 + 1, which is 1.
Then the backdrop colour is set to (1,1,1)
On the second run of the loop, r is set to 0 again, then 0+1, and the backdrop colour stays at (1,1,1).
To fix, move r=0, b=0 and g=0 above your While loop. :> Then they will be run once, but not again.