During the rain, Text color in the chat goes gray. When I am holding a weapon or tool, text color goes white.

Not sure what makes you think this is a bug in Ex Nihilo Creatio.

Can you reproduce this error with only Ex Nihilo Creatio installed? Otherwise I suspect it is another mod; do you have any mods which changes rendering?

What other mods do you have installed?

I tried to reproduce this error with only Ex Nihilo mod, I didn't found this error.
I found out that Durability Show mod has this error.

I suggest reporting this issue to the affected mod.