The description of veils only has it duplicating the individual 'layers' of a prismatic wall, not the effect of a prismatic wall spell. Whether this allows them to be dispelled or not, I don't know, but I think so.
It's a bad idea, though, cause of the DC's involved. Dispels are already checks that favor the defenders, and with the +DC from the ISV class, and the +DC from the ring of enduring arcana he's probably wearing, and the +CL from all the other items/feats/spells he's got up at any given time (like any other full caster), all paired with the cap on every version of dispel, you're probably looking at (at best) an 18 or better needed to roll on a spell theft (if not just flat out impossible), and probably some similarly difficult numbers even with reaving dispel.
I mostly threw that out there as sarcasm.