When two traits are mixed together like paint to create a third phenotype

This is called incomplete dominance.

Explanation: Incomplete dominance pattern of inheritance is an inheritance pattern in which two alleles combine together to form a third phenotype that is entirely different from the phenotypes of the two alleles.

Incomplete dominance is a heterozygous condition in which the two alleles at a gene locus are expressed in the third phenotype that is a blend of the phenotypes of the two alleles. In incomplete dominance, one allele is not fully expressed over the other allele that is none of the alleles is fully dominant over the other.

Example of incomplete dominance occurs in rose flowers in which red roses (RR) are crossed with white roses (rr) to produce pink roses (Rr).