

They probably did. It’s not exactly honest, but the system is technically outputting a colour signal, and it was released at a time when that wasn’t a given. They didn’t say “full colour” anywhere on the box, did they?
Let’s call it a mix of lower expectations for the time, and a bit of marketing deception.
These “home pong” consoles were very common at the time. They don’t really do much, so their main value is historical interest, and this isn’t a particularly famous model. A quick eBay search seems to indicate it might go for GBP 80 at most, but probably more like GBP 20-40. So OP got a good deal, but they didn’t find a lost Vermeer. :)