dont be that guy, if a store had to post the return policy of every item, you wouldnt be able to find anything, just policies.
plus, you chose the paint. why would they take it back and take a hit on the cost..
Nothing wrong with being "that guy". Most states require stores post their return policy. It's not that hard to list the items which are exceptions to the normal return policy.
Stores have a choice. They can clearly list the return policy or they can accept returns from customers who insist a store adhere to the posted policy.
Being "that guy" may motivate a store to change their signage so their intended return policy matches the posted return policy.
Many stores accept returns, even though the merchandise can no longer be sold as new merchandise. Stores take a hit for goodwill. Improve repeat business.
Other posters made a good point. See if the store can tint your mixed paint in order to give you a color you can live with.