Axolotl Colours

When do axolotl hatchlings begin to show their actual colours? How many days?

It takes a while before you can REALLY see a lot of color difference. When they're babies you can extrapolate the color but it's not exact until they get bigger.
Here's some tips for the babies.
Try shining a light in their eyes to differentiate melanoids. The melanoids won't have a shiny ring around their eyes. The leucistcs are just white with the shining ring. Wild type are pretty easy because they're pretty green with obvious spots. Albinos eyes give them away and it's easy to differentiate a golden from a white albino just from skin color.
Hope this helps!

It really depends on the color.

However just prior to hatching you can tell Dark (wildtype/melanoid) from Light (leucistic/albinos)

To tell the diffrence between wildtype and melanoid you'll have to wait a couple of weeks, wildtypes will have a noticable spotting pattern, melanoids will have no to virtually no spotting at all and be a solid color.

To tell the lucy's from the other albinos is easy since they will have black spot eyes.
Telling the albinos apart from eachother is a little trickier and you usually need to wait a few more weeks.

As suggested you can shine a light on the eyes to see if you have an albino-melanoid.
the golden color of the xanthic will become evident pretty quickly too.

I you are looking to tell if any are GFP the best bet is dark room, I use a handheld black light I bought off ebay.
and I sort by first shining a light in the eyes, and then going back and resorting by shining a light on the belly. i've had a few that I mistook for non-gfp so now I use the belly trick.