Not sure what your problem is as you have not explained it very well, but i think your problem is using luxrender did not give the results you wanted, and i think your solution would be to used AOVs in luxrender which can put just the caustics on a separate layer then in the blender compositor or photoshop etc you can take just the caustics and do ANYTHING with them, color them, blur them, darken/lighten them, or anything else you can do with compositing software and then mix it seamlessly back into the final render.