timeshift bar - what do the different colors mean?

#1
I see 4 different color in my bar, but i cant fint any explanation for them. Green is clear - thats the used part, white is the empty part, but there is also grey instead of grey in some parts of the bar and sometimes also light green instead of green in parts of the bar. i can guess a bit, judging from the behavior, but i'd rather have a clear explanation :)

i use the 32 bit version on x64 windows, whole buffer (1.2GB) is in memory.