So, I didn't understand the problem. Sorry, I didn't see the game, so I can't compare umodel screenshot with the game. I'll try to "predict". You're seeing greyscale texture in umodel, but game has the same texture colorized? If so, there are 2 possibilities:
1) the material (not texture) is colorized with shader using some special texture or some formula
2) the mesh has vertex colors
(both variants are not supported by umodel)
Hope you've had a very merry Christmas and will have a happy new year
Thank you! You too