Hello I have a problem is that when I play and try to build a weapon or suit the skin of equipment cubes become transparent. I also appears transparent color mixer and do not know how to fix it. Can you help me.

Does this modpack have FastCraft or Opitfine?

yes optifine

Try removing Opitfine and see if the issue still happens

Mod list/mod pack please.

Thank a remove optifine and the problem solved