I have not heard of photon density as you put it.
What you are referring to are two different things.
A violet laser, and a red laser are placed next to each other. The red laser is pointed at an LPM and the reading is 5.000mws. Next, the violet laser is measured to be 5.000mws.
Each laser is capable of doing the same amount of "work" as the other. The difference, is that each violet photon carries with it, more energy than any red photon.
So yes, 5mws is 5mws. The violet laser has fewer photons, or energy "packets" than the red laser has.
Visibility is something completely different. Our eyes pick up the color spectrum in a rough sort of bell curve from violet to red.
5mws of green in a sense, will appear MUCH brighter than 5mws of violet, red, or blue etc.
Hope this makes sense for ya..