Quick bit of advice needed. My first paid painting job ! Customer bought some deep yellow paint...had it mixed so can't take it back. Decided it was too bright. It's a Dulux vinyl matt. Can I tone it down by adding white to it. I've got some Mcphersons matt but it's not a vinyl matt.... would that cause a problem. Secondly I've got some paintwork to do inside. Am I ok to use Johnstones Primer / Undercoat which is water based followed by a Johnstones gloss which is oil based? Could there be a reaction ? ( the primer / undercoat goes on so easily compared to the oil based u/coat ) Thanks for any advice.