Eight percent of the male population suffer from some form of colorblindness. Therefore the default choice of colors for download and upload rates in gnome-system-monitor is not so good. I suffer from mild colorblindness, and have problems separating colors with a blue base and red or green tints from each other. It took me quite some time to discover that you can change the default colors, and up to that I had to guess which was upload and download. It would be better to switch one of the colors to something very different, for example dark red or brown, and keeping the other one blue. 

I sincerely hope you perform this easy fix!