Roan coat color in cattle develops through incomplete dominance of the gene for red color and its allele for white. what crossing combination will yield a herd of true-breeding, roan-colored cattle?

Roan is a combination of white hairs with red hairs. When we see cattle like this, they look pink because it resulted from a mix of white and red.

Red allele: A
White allele: a

A heterozygotic animal has this genotype: Aa
Like i said previously this is the mixture of colors so the animal would be pink.

A red coat color has this genotype: AA
A white coat color has this genotype: aa

To get all the herd to be roan-colored cattle the combination of parents should be: AA x aa. -> 100% roan
Another possible way to obtain would beroan-colored cattle: Aa x Aa or aa x Aa (in this case there would be less probabable to obtain road).