What he said.
Yours is a juvie... most likely 6inches or less. If you want him to change color, change the tank surroundings. I'd say your tank is well lit. Darken it(long term= weeks or months not hours) and watch him get darker.
There are at least 3 different types of plecos sold as a common.
A Hypostomus Plecostomus is now the hardest to find which is a brownish pleco.
Then there is a P.Pardalis and a P.Disjunctivus. These two are harder to tell apart. If I was to throw a guess out there, I'd say It's a Disjunctivus. Again, wait till he grows some more to tell for sure.