That has probably to do with the program with which you read your email. The coloring is done in a cascading style sheet, which the email program has to load from the elog server upon display. Most programs ask you if you like to "load images" for this email. This is a bit misleading since the style sheet is no image, but similarly loaded form an external source. I use Thunderbird, which remembers my decisions. So if I get an elog message from a certain email sender, and I click on "load images always from this sender", then it will show the colored background for each following entry. If the sender now is different (like if you use a different computer), this will be different and you have to load images once by clicking somewhere (mostly a bar above your email display).