cattle hair color shows codominance. red is dominant, and white is recessive. in the heterozygous condition codominance is shown as a roan (something of a light red) coat color.
four o’clock plant also known as mirabilis jalapa. not sure where you get the ‘japanese’ though. anyway it also shows codominance. again red is dominant, white is recessive. in the heterozygous condition codominance is shown as pink in colour.
Probably too late but …
Actually, the four o’clocks display what is called Incomplete Dominance (where only one of the two alleles contributes to the phenotype), and the heterozygote is intermediate in phenotype due to a dosage effect — it only makes half as much pigment as its homozygous pigmented parent.
Codominance is where both alleles (e.g., wildtype and mutant) produce a gene product, again giving an intermediate phenotype but as a result of "blending" (with apologies to Mendel).
Incomplete Dominance and Codominance can be very difficult to distinguish at the phenotype level — may need molecular/biochemical information to be absolutely sure.
I thought roan meant two two hair colors are expressed simultaneously (e.g. black and white hair), instead of grey hair. But then I wouldn’t understand, each heterozygous cell would grow multiple hairs? Just don’t get it…