Problems with painting Drywall

Hi
I have an area in my house where the previous owner installed drywall, and I just hired someone to finish off what the prev. owner started, and install the rest of the necessary drywall. The person I hired was supposed to paint all of the drywall, old and new, in white, using a primer + coat for the first layer, and white for the second. He claims to have done that, but the older drywalls are yellow, while the new ones are white. He wants extra for a 3rd coat and says this sometimes happens with old drywall.
Is he lying?

Hi

First of all if the old drywall was properly sanded down it might have helped a bit.

It also depends on the quality of the paints been used. A cheaper brand pva might encourage yellowness while a higher quality sheen may have covered in 2 coats.

Conclusion
If he used a pva he might not be lying.
www.mor-micronpaints.co.za

Bertie