Painting on cement internal wall - help!

[making a mess]

Hi, new poster so be gentle. . .

Just had a wood burning stove fitted and I am trying to paint over the new inset. The fitters simply said to apply a few coats but having done that the new surface is a very different colour to the rest of the wall. It seems the wall around inset is absorbing much more water than the rest of the wall. I suspect they used a cement/plaster mix. . .

Having read some more it seems we should have applied a cement bonding?? coat or something. Any ideas how we should proceed? Is it too late to apply a cement coat and just repaint?

Don't want to make any more of a mess.

 

[footprints]

Sounds like one area is absorbing more paint than the other or has a different texture to the other so will reflect light differently.
I would try a sealer like Zinsser Bulls eye 123 then repaint at least a couple of top coats using the same tool (brush, roller, pad or spray) that the surrounding wall was done with to try and achieve the same finish.
A full coat over the lot will help too, as paint colour changes over time and touch ups often do not follow true to the original paint.
Almost impossible to completely hide a repair like that but should end up a reasonable finish.