Cause
This colour variation is highlighted where there is a difference in thermal conductivity i.e. the joists and the voids between. Or on internal walls of a timber frame construction highlighting the timber or steel upright studs.
Pattern staining has also occurred where cold water pipes have been chased into the wall before plastering.
Remedy
A paint coating can only improve the aesthetic appearance. To solve the problem the voids between the joists need to be filled with glass fibre or some other type of insulating material .
The illustration showing the pipework can only be rectified by re directing of lagging the pipe.