Not sure if you can just add paint but i would definately bring it back to the shop for advise first because a) its acrylic paint so anything added would have to be the same and b) the paint is too expensive to mess about and let it go wrong. Also the shop will have the colortrend machine to mix any addition through properly.
I painted one wall on the dining area of my kitchen Colortrend Select I love it but took a long time and a fortune in colour samples to get the one i liked. Gooseberry actually goes on much darker to what its like when it dries, so if you do try it, dont freak out initially! Prefered the select range to the DeVine reange myself - paint went on nicer, and you can get the select range in matt or sheen - think i went for matt but couldn't be 100% till i check. Fab paint brand though!