Hi there,
So I have them both and the 2.9" quad color. I found the same below a certain freq it’s lighter. I think it’s the LIB standby and let Seeed Investigate. I’m going to look at it and post some side by side pic’s see if we can shake the tree of knowledge 
Pretty sure there are other adjustments they can make with the LIB Have you tried any other BSP’s for the C6, Do you know which is used in the example you are using? LMK, P
HTH
GL 
PJ 
Wait that is NOT a seeed Display? is it?