Custom field accordign to the product in a category

I'll make it simple
I am selling 2 Tshirt
One available in black and white, so i create the black and white custom field, assign to the category Tshirt
But I have another tshirt only available in pink and green, also in the Tshirt category , so how do i assign the custom field green/pink
You work the other way as most of the cart, meaning, a product has always the same custom field assign , accrfing to the category
I need in my product detail to assign the required custom field
thanks

Thanks for your anser
it is just horibly complicated for my basic marketplace , nobody will figure out how to use it