Separate colors for each chat line, not user.
Change the color preference in chat, which shows the username in a single static color to be a color per chat line.
This means that there will be 4 colorblind and dark theme color friendly colors that appear for each username per chat line.
User 1 (in purple): blah blah blah
User 2 (in blue): blah blah blah
User 1 (in orange): blah blah blah
User 3 (in white or dark grey: blah blah blah