Unfortunately all my fault, bought and chose it alone. It's a standard ready made colour. I could say the poor lighting in B&Q made their test piece look like the soft minty green I wanted rather than the lurid green slime it comes out on the wall :-)
AS mazie says you can buy tiny trial tins of Dulux paints, you buy one and then try it on a test area at home where you want to use it so you can see the final effect before you buy a big tin. Also as O_G says some colours look different once they dry.