Flory wash is just a powder or pastel in a water soap suspension. It is intended to go into surface details and panel lines. Using it to "tone" down a decal will end badly, trust me on this. Use the wash to highlight panel lines etc. Then when cleaned up you can flat coat the heli, then use pastel chalk on a paint brush to "dull down or dirty up" that decal. This will work like you are wanting, the flory wash will ruin the decal and you won't be able to fix it.
BK