Try find out what the route of the problem could be? It could be something such as the time spent colouring-in in class is too structured for your LO. He might not be developmentally ready for that much structure and conformity.
Or as the other ladies have said the teacher may be correcting him and this has created an issue surrounding the activity.
Try make it as fun as possible, remove the idea of colouring-in for now. Get down on the floor on his level and put some big paper out for yourself and put one in front of yours for him but dont ask him to come over. (Maybe draw circles and squiggles on the pages with black koki). Draw on your page and colour in the design. Tell him what you are doing. "Oooh, what colour do you think would look good in here?" "What could I draw in this space?" "I like the way I have done this section. What do you think?" "I dont like like this colour that much." Speak to him a lot to get him interested in what you are doing even if he doesnt join in at first. If he wants to join tell him you have a spare page he can draw on if he wants. Use as many opened ended questions as possible, that way the issue should come up. Make it as positive and as fun as possible. But NEVER force him to join in.
You can slowly move back to more structured pictures. But there is no real developmental reason for children to colour-in as such. they need practice with pens and crayons, for pre-writing skills. So do as many drawing activities as possible. He should want to colour-in his own creations. Again say to him what colour can the 'alien' be? prompt him to do it with out forcing, If he wants to praise but dont over do it. If he doesn't want to just accept.