What if you don’t like the color you chose? Can you take it back to add a color to change it? For instance, I chose a semi-gloss yellow for my bathroom and realize after putting some on the wall that it is too lemony. Can I add a touch of green or brown to change the color a bit?
The answer to your questions depends entirely upon where you bought the paint.
A good paint dealer will easily adjust the color to more suit your desires. They may advise you they will not be responsible for the final color, but given good information a good dealer will be able to alter the color for you.
On the other hand, if the paint from a warehouse type store, I would suggest buying paint the color you prefer.