I've had 2 instances of Valspar paint not matching.
In the first instance, I bought a gallon of paint at store A. After doing all the trim and 3 of the walls, I ran out of paint. Went to store B for gallon 2. Finished job but when it dried, it didn't match gallon 1. There was a significant difference in shades. Took some pictures, went back to store 2. They agreed paint did not match but suggested that I go back to store 1 since they sometimes have "tinting issues". They mixed the paint at store 1, didn't charge me. I applied gallon 3 to a small area...it matched gallon 2. Instead of fighting them, I repainted all the trim and the other 3 walls. Lots of extra work but it looks great.
Instance 2. This time I bought 2 gallons to paint a room of smaller size than first room. Even though I was painting over white walls (with a light yellow paint), one coat was not enough. I only had about 1 quart left after one coat. When I ran out, I bought the 3rd gallon. Guess what??? It, too, doesn't match the first 2 gallons and this time I went to the same store (they even used the same machine).
I don't know who to turn to or what to do. Any suggestions?