This probably belongs in my mantis color variation thread, but since this is more specific, I'll post it here.
Ok, so I've been keeping 2 spiny flower mantids with colored flowers. One with rose petals glued to the side of it's container, and 1 with a bunch of pink flowers from some tree.
Before I begin, forgive me for being anal, Katie, but I believe that is at L4. L1 hatchlings are all black, L2 get their first white spots, L3 is when their abdominal spines are first visible, and L4 nymphs' spines and coloration become more prominent.
I've noticed that nymphs will only change color at L4(like you have), but it happens over a span of time, like say a week of color exposure, not between molts. Someone else described this keeping a spiny flower on a rose in the sun. Also, the nymph I was keeping with pink flowers turned VERY pink in it's face, sides of the abdomen, and 2 of the 3 foreleg segments. YAY! No change on the one with rose petals, but to be fair, it's still at L3 and the petals are old.
Also, some nymphs will be more colorful than others, in a plain environment.
Keep us updated on this, because it will verify a theory I have that mantises eat bits of flower and store the pigment in themselves to change color. I came up with this because of a pic I saw of an orchid mantis eating the side of a petal. I also removed the pink mantid from the flowers to see if it will change back to normal. And I plan to buy some yellow, red, and blue flowers to see if they can turn those colors