Oh dear that's not a good sign! Whatever happened to the customer is always right?
I can understand resistance to white mat emulsion as it will probably take more coats to cover.
Please bear in mind they will use 'Trade' emulsion which has better coverage and allows the drylining to continue to dry out.
What this means is if you re paint with vinyl matt, which does not wash off, you may be sealing in moisture.
Provided you don't move in December etc you should be able to re paint he walls within a few months.
It appears the Keepmoat financial end of year is 31 March, so you should have an issue of your home being rushed to be include in year-end figures.
Better, that Keepmoat paint them the colour you want, white, at the outset before carpets, curtains and furniture makes the job harder!
It will also take at least tow think coats of vinyl matt white emulsion to cover the horrible magnolia most builders use.
Some of the better housebuilders paint walls white as standard, because it is more classy.
I forgot to add, get your home professionally snagged and inspected before your legally complete.
I suggest Time Fee of Snagging Inspections 07768 900200
Keepmoat don't have a very good reputation for quality and customer care - 30% of their buyers only rated them four stars.
