Their confidence/comfort level has a huge impact on how dark their colors are... the more confident/comfortable they are the darker they become...
So the Dominant Dempsey in a tank will be the darkest... I've heard people say that dark means stress, although this is the complete opposite of my experience with many many Dempseys.
They usually start "coloring up" (blues & reds) around sexual maturity (3.5~4") and it continues to increase until around 6"... by the 6" mark this is usually what their colors will look like for life (allowing 'darkness' to vary with confidence).
I've seen a very large range of colorations in Dempseys and it seems the lighter/paler end of that scale is stretching further and further as the years go on. I would assume this is due to them being more and more generations from wild caught.