Few days ago I have found this video (English subtitels) about color reference chart:
something like grey card above, but with more options. From 2m35s in this film you can see use of it with video editing software and Vectorscope. Is someone using Vectorscope in kdenlive and can explaine some more how to calibrate video in kdenlive to such Reference Chart? As we can see from 4:00 the results of proper use those tools are huge.