I agree that it's likely just colouring, looks pretty healthy from the photo!
There are quite a variety of sub-species of otocinclus, with variations in size, colouring and markings, and usually just sold as "otocinclus" unless it's a standout difference like the zebra otos. I bought a batch of six or eight (can't remember how many I bought now) at the same time from the same tank, and ended up with three different types of oto in that one batch. Have to look closely and research the differences to really notice, but it's an interesting rabbit hole!
Especially when you learn that certain otos are often found living near/among certain corydoras species... There's a theory that there's some mimicking going on, since cories have more defences than otos, it would certainly make sense. Yeah, fun rabbit hole! Awesome little fish.
I agree that is no algae on your oto. And that there are several mixed species marketed as common oto.
Regardless, your photo is of excellent quality. Good for you!