Yeh, I sold one that was about 15cm for £40, I didn't particularly like it, which Is why I sold it. I've still got 7, I'd like more. Unungy has a wholesale offer of 50 for $2500 which really got my mind going. I'd love to see a big tank full of wood with 50 little buggers in it.
I think, in general, yes, their colour does improve with age, but it's also dependent on several factors. I have one Xingu who I would consider a very high quality with bright yellow 90% of the time, and rest falling into lower percentages. Being active boldens their colour, but the norm for most of them is a dull browny yellow when they're just still. Getting them used to full lights helps, but takes ages for them to get comfortable under them. When I take pictures with the lights on, it's only the boldest that come out often and their colour is usually brighter. For the past 6 months or so I've been using the lights more and now the majority are relatively comfortable openly feeding. But the best way, and my favourite way. Is with lids up and room halogen spotlights on, like I am as I write this. I can see 8 of the 12 in the tank right infront of me, but I have been sitting here for the past hour or so.