Paint Match Help!

[mummyfahey1234]

I've bought magnolia paint to touch up the walls in my rental property which I move out of this weekend but now I've put the first coat on the original looks more cream :( how can I get a definite match ( I know different brand will do different shades) b&q does a colour match but you need quite a large sample and there's no way of me doing that. Help!! I'm fretting right now