@blackyps The reason I suggested an automated message rather than automated restrictions is that as this now massive thread demonstrates the choice of colours is very contentious - some people want more variety (e.g. to be able to choose from lots of similar shades of the same colour), some people would favour less variety in order to increase contrast between colours, and some people want things to stay the same (which is now looking by far the most likely outcome given the lack of consensus).
Personally I'd prefer an automatic restriction like you mentioned where if a colour is chosen you can't choose another similar colour, but that clearly disadvantages people who feel strongly about being a particular colour no matter how similar to another player. Since there's no overwhelming majority in favour of a 'reduce colours to improve contrast' approach, I thought a prompt could be something that doesnt disadvantage anyone (with the exception of dev time required to code it in, and I have no idea how much or little that would be), and so might be an agreeable compromise that is an overall improvement if it was a low effort enhancement.
It's effectively applying a nudge theory/principle - you prompt people to change the colour, without forcing it. Those who feel strongly about being the colour they want dont have to comply. Those who don't mind are likely to change if requested by someone who does mind. Those who prefer no similar colours but don't think to check (myself included) would similarly change to avoid a clash.