How to bleach and re-dye a viscose/elastane dress?

Bleach will destroy the elastic quality of the dress in time. But as a one time thing it might help but it may not bleach out evenly.. Try to dye it a dark color first like the colors you mentioned redye it until it is as dark as you want it in strong light the pattern may still be visible but it will look more lick a texture than a pattern.