Which really don't mark down on colour choices.
But they do mark ERF seats down. This is because they consider them difficult to install.
It's tricky really - is it difficult to install? Compared to the Cybex, yes, significantly. Compared to other ERF seats of the same type, not at all - it's perfectly standard.
If you're willing to follow instructions, maybe watch a video, ideally get someone to show you in person (tricky at the moment) and generally just make sure you've got the installation right, then it's likely that "difficult to install" isn't a factor that's relevant to you, and what you should really look at is the safety rating.
What Which? aim to do with their reviews is present a list that any layperson, with no previous knowledge or interest in car seats (or whichever item) can pick up, purchase an item and get a reasonably good performance from it. If there's a high chance that this person will install the car seat incorrectly, because they haven't read the manual and it has parts that they don't know what to do with (Swedish style tethers) then they aren't going to get a good performance from the item, in fact incorrect installation could be incredibly dangerous.
When Britax first started selling these seats on the UK market, even then they had a disclaimer on the website explaining that they would not sell them online, only through specific approved retailers who had training, as the risk of someone misusing the seat through incorrect installation was (they felt) high. It's also why they only tend to be sold through specialist retailers, and not in places like Smyths and Halfords, even though they stock other Britax car seats. It's different in Sweden, because this is a totally standard type of car seat and it's the cultural norm so most people would know how to do it, or be able to ask a car seat shop, garage or other parent for help. But you show this type of seat to someone in the UK that doesn't know anything about ERF and has never come across tethers before, and they will typically ignore the tethers altogether, which would make the seat very unstable due to the higher weight and size of child it's designed to accommodate. And then it's not the safest seat any more.
I would say that the equal safest seats on the market at the moment are
Britax Max Way Plus
Axkid Move
Axkid Minikid
Besafe izi Plus
As these are all plus tested but also have great side impact safety.
They all install in a similar way, so do make sure you're familiar with how tethers work and/or are happy to call the in car safety centre back for advice if you get stuck - they are really good.
Cost-wise, Axkid Move and Max Way Plus are closer to the Joie Bold whereas Minikid and Besafe are twice the price.