How much can you grind off of the end of an LED without hurting it?

Good question. I don't know for sure whether the length of the lens has any bearing on color or not. I don't think so.

Another issue might be heat transfer through the resin that might affect the color..again I don't really know.

(I had 2 blue LEDs, both from RatShack that were supposedly the same....unfortunately I didn't A/B them before grinding, but they are definately different hues now, and I have no idea whether the grinding might have caused this, or just Ratshack being cheap with QC and sourcing!)

Anyway, you should be able to see the semiconductor emitter through the lens. As long as you have a visible layer of epoxy still over that you should be okay.

The bigger issue is that you will need to polish the resin after grinding or the light output csn be attenuated quite substantially. So make sure you have the means to do this (dremels are great things).

Try the blob without grinding the LEDs first if there is room. The silicon is definately more effective than the grinding. And it might be worth investing in a couple of LEDs for practice too