I wouldn't recommend using the burn tool in this case.
Make your selection using color range if you like, then create a Black & White adjustment layer and a Curves adjustment layer. The Black & White adjustment layer should use your selection as a mask, you can then copy that layer mask onto the Curves adjustment layer. If the transitions between the adjustments are too harsh then you can 'feather' your mask after the fact by selecting the layer mask and using the Gaussian Blur filter to ease the transitions between black (unaffected) and white (affected).
Reduce the Fill on the Black & White layer significantly to something like 20% (at 100% it will completely desaturate the roses, at 0% it will have no effect). You can then play with your Curves adjustment layer to change the red/green/blue levels in the roses.
You could also use a Hue/Saturation adjustment layer if you want to adjust the hue that way, but I find that Black & White is much better for adjusting saturation.
An alternative method might be to use the layer mask created in the above process on a dark red solid layer, set its blending mode to Multiply and reduce its Fill %.
Darby Edelen Lead Designer Left Coast Digital Santa Cruz, CA