The coloring looks quite good actually. I wonder how Triton looks in natural colors in the first place, Voyager's camera didn't have the right color filters to easily compose true color images.
As usual, here is my take: https://fargetanik.deviantart.com/art/Triton-Texture-Map-14k-622030522
I left the unimages areas blanc and tried to make a smooth transition. If you want to fill fictional details, you should use a transition with transparency. I am using Gimp, here is what I usually do:
Let's say you have a layer with the blanc areas cut out. Deselect every color channel except for transpareny, then use gaussian blurr with a high radius (will eat a lot of processing power!), then use the alpha channel in the histogram and cut out the lower half of transparency values.
Here is a good take on a map filled with fictional details: https://kexitt.deviantart.com/art/Triton-maps-673821587