Red implies problems, green implies "normality", but here this association is misplaced. Perhaps a typical "fire" palette would be better - from dark brown to red to orange to yellow and, ultimately, to white for the extremes.
In the meantime, it's very easy to tune the colors your own: just modify this line https://github.com/draios/sysdig/blob/master/userspace/sysdi... in your local version of the script, using this as a reference http://misc.flogisoft.com/_media/bash/colors_format/256_colo....
I believe the issue raised isnt the palette range itself, but rather that it is the reverse of what it is typically expected. The current red area "should" be green indicating there are many calls in the fast region while the current trailing green blocks "should" be red indicating problem issues
This color of green=good and red=bad I believe stems from Triage tags: http://en.wikipedia.org/wiki/Triage_tag
Sometimes white is used below green as 'dismiss/not an issue'