i have had harlequin rasboras get nice and red. i think it has to do with dominance within the school along with water quality, and overall stress from other fish.
example, i kept 5 in a 10 gallon tank, 1 was always a deep red, another had red but not nearly as intense, and the rest were plain. i moved these fish into a 75 gallon tank heavily stocked with other fish, now they are all a light pink, but none show the red. i think it is because they are slightly stressed from the other larger fish in the tank. while in the 10 gal tank, the dominant fish displayed the best colors.
29 planted biocube, 2.4 gal shrimp tank.
80 gallon Reef tank
lots of clownfish.
1 crested gecko
12 inch tiger salamander