How to avoid this effect, and is it possible to reduce it to 25%
Thanx
leon
its called color bleed. and usually a: modeling to scale helps some. but mainly b: use the saturation spinner in your indirect illumination rollout. put it to something like 0.3
c: Right clic then put down the emitting GI in the Vray property of your object, the advantage is to not loose contrast in your image between the sat spinner.
Hope this can help you 
or as ive been doing recently i have a floor riiiight under that floor which i make white. i think i put a post about it here in render theory or in tips n tricks
You could also put a vraymtl wrapper around it and reduce the emmitted GI output - prettymuch the same as what Priad said.