FuriousWeasel that is exactly what I thought about when I saw the name of the save.
Thanks. I am added that metl must be placed before filt. I didnt noticed it because I am using script for layering.
See on step 2, and You see what i mean.
You miss something. You need to make FILT's index > METL's index. Because when particles are overlapping, upper particle have more index value than overlapped particle.
Unless you intend on having the buttons change colour. To get the same effect: You can just paint a small portion of desired material a color, then use the "Smudge tool" to get semi-transparent colouring. Then Ctrl+C & Ctrl+V the desired semi-transparent colouring.