Personally, I find this to be cleaner:
$cl = sprintf 'color:%s;', $c_diff <= 0 ? $color_good : $color_bad;
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.