why are there still some colonies growing in the areas exposed to ultraviolet?

Answer 1

I assume you are looking at a Petri dish ?

Well not all bacteria is effected to the same extent (see eg. u.v. resistant Bacillus subtilis).

Answer 2

some people arent affected by it as much as others so they do not have to escape it like others....some people can stand it and there fore will have more room in their land and less competition for resources