Is it personalized? Because I can't see why some are colored and some maintain the default color. If it is, then maybe I have visited the colored sites at some point(I don't have an account to half of them, but hot cross questions are attracting)?
Most of those colors correspond to the color of the top bar of those sites, which all have a custom design. There are some exceptions, maybe because of the design overhaul.
E.g. Raspberry Pi:
and Movies & TV:
If it's not the top bar color, it's another color which resurfaces in the design of the site.
The colors are the same for everybody and do not depend on whether you have an account or not.