Neocaridina color is tricky. The blue coloration in blue velvets is similar to that of blue pearls - it's translucent, rather than opaque. Fire red and fire yellow neocaridina have solid, opaque coloring that you can't see through. These guys were selectively bred so that the coloring is more pervasive as opposed to darker, or at least so it seems from photos and my experience with RCS (they throw a "painted fire red" female every once and a while).
A lot of neocaridina coloration has to do with the background they're sitting on. That said, I really haven't seen blue velvets of significantly different coloration sitting next to each other on the same substrate in the same photo. You can't go with different photos from different people and different tanks to say that one is darker or not because their color is so environmental and the white balance of the camera could easily be inaccurate.
So to sum up: good question; I dunno.