Yeah, I agree that needs improvement.
Out of interest, does it change if you enable High Contrast in Labs?
As mentioned before, if you have an account or pot with £0 balance, select that too (so you are viewing the Joint account and the £0 account/pot) and you’ll get a different coloured line.
If you don’t have an account/pot with £0 in it, create a temporary pot and keep it at £0