Do the colors breed true? Folks call the mixed color chicks mixed breeds, but they are all Orpington....
There is a lot of debate about that. I'm not sure which country you are in, I suspect the UK. The UK and US have different Standards of Perfection (SOP) which is basically what defines a breed in those countries. There usually are some differences in the countries' SOP's. I often say that breed is a manmade thing. What is a breed in one country can look quite different from what is called the same breed name in another country.
The SOP defines why an Orpington is an Orpington. Body shape, posture, leg color, ear lobe color, comb type, weight of the adults, types of feathering (not colors), shape of the tail, number of toes, the list goes on and on. A good judge can tell breed of a chicken that meets the standards by it's silhouette, shape is that important.
In the US, the approved Orpington colors/patterns are Blue, Black, Buff, and White. Some people are pretty firm that only these colors are Orpington in the US, any other color or pattern is just a mutt. Others say that as long as they meet the requirements of the breed the color really doesn't matter. That color just hasn't been accepted yet. I don't show chickens but some people on this forum that do say you can show a bird as a specific breed even if it is not an approved color/pattern. That might depend on the show. If you are showing it as a color it needs to be that color. That's why they approve colors.
Any time you get people together, especially people that are passionate about something, you are going to get different opinions. I don't breed to the SOP anyway so I don't worry about what people think about it.
My suggestion is to choose the definition that makes you happy. You might as well, no matter what you choose some people will agree with you and some won't.