The top right corner of the screen has the trigger channel indicated in the correct color, along with a symbol indicating type of trigger, and the trigger level.
If the trigger indicators on the edges of the trace window were the same color as the trigger channel, some people would probably complain that they are confusing them with the Channel baseline indicators.
I even saw a YT video just today where some fellow was complaining about the colors on the DS1054z, and the way he complained about them made me realize that he had a certain type of color-blindness (tritanomaly) and probably didn't realize it himself! He was also slapping the buttons around and twisting knobs like they were faucet handles and complaining about the responsiveness.