Thank you for the compliments

I don't do anything special in particular to maintain their colours. Like most people here, I keep an eye on my water quality, I perform regular water changes (currently 20-30% twice a week, because my tank is overstocked whilst I plan my 540 litre purchase) and feed a varied diet. They were not as bright as this when I bought them, so hopefully I am doing something right!
My maintained parameters are
Ammonia - 0
Nitrite - 0
Nitrate - 15-20 (my tap water reads 10ppm and my tank is planted so this level is maintained)
PH - 7.2 (originally closer to 8, but I use RO water for 25% of the volume)
GH - 200mg/l (originally 300mg/l but RO water brought this down for me)
KH - 110 mg/l (was originally 160 until RO water)
Phosphorous - between 2 and 4 (2 from the tap)
I have fairly hard and alkaline water, enough that limescale damaged my filter impellor within three months of purchase. This was the main reason I decided to soften the water, since my fish were previously happy in the higher readings anyhow.
Shrimpster - mine happily take bloodworm from my hand lol!