the title

in fact the actual name in persian "Rang e khoda" means "The color of God". It is because the boy is searching for an explanation of why everyone is talking about God and he has heard that God speaks to its creatures. So throughout the film he tries to read the signs everywhere which would lead him to find God, so that he could tell God the things he has in his heart. and at the end the light in his hand may give him the answer.

but isn't there a reference between mohammed and the carpenter about the color of paradise?

When I heard first about the movie 6 years ago I thought it was titled the colour of god and I think this is the original title.

The last scene, always in Madjidi's films depicts a conclusion, like a paper. You know the child was blind; he just could touch thinks, read from touching words by fingers and so on. The main aim is to find the answer of a child in his innocent world, where is the powerful thing, which really loves him, despite his disability, the god. I think, the final scene wants to tell us that the blind boy will touch the god, despite his disability, the color of god was touched by Muhammed and whether he survive or not, he got his answer.
Also, the title want to make us aware that the color, for a blind boy, should not be seen, it can be touched. And for us, what is the meaning of the color of god or paradise?