Inconsistent color on rendered image Clearly because of lightning but not sure what is the best way to fix this. Suggestions?

Edit:
The problem is with area in the red. The color is really inconsistent (Looks like clouds?). Lightning source would fix this but how would i achieve this naturally. Or its not achievable? 

Material on walls, roof and cabinets are just normal diffuse. Tried to add more light bounces but that didn't help.
Do you have the De-noising filter on?  If so you might consider increasing the radius.  See this answer also: How to reduce noise in really dark scene