Par is generally considered from 400 to 700nm. Actinic is 420 and still within the range of PAR.
This graph is for land based plants but it kinda gives you an idea of which colors are utalized the most.
I figure everything would be shifted towards the blue end for corals and continue moving towards blue the deeper the coral was in the water. I figure acros and other shallow water corals should have quite a bit of red and yellow light. One thing to keep in mind is the 20K halide only puts out about 40 to 50% of the PAR a 10K does. It would be interesting to do a long term experiment running 20K 250's with 10K 150's to see what happens. You wouldn't have to worry about burning the corals with too much red and yellow from the 150's and the 250's would provide plenty of blue light for coloring.