Here's two additional interesting quotes about the genetics of the cremello trait in horses:

Chapter 4.13 - reproductive genetics, page 129
"Occasionally it may be possible to differentiate homozygous and heterozygous genotypes by observation of phenotype. Another coat-color trait can provide an example of this. The palomino gene (Ccr) causes a distinctive golden hair color by dilution of the red pigment to yellow, and is highly prized by many breeders. Homozygotes (CcrCcr) for the gene exhibit a "dosage effect" in that the coat is diluted to a very pale creme (cremello), which is usually considered undesirable. "

Siegal, Mordecai, ed. UC Davis Book of Horses. 1997. Ch. 4, section 13, p. 129.

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"The C or color gene is necessary in dominant form for the expression of any coat color. A homozygous pair of recessive c genes (cc) will result in a true albino. The recessive c gene is rare and may not exist in the gene pools of most solid colored breeds. the c(cr) gene acts to dilute the red pigment to yellow in the heterozygous form (Cc(cr)) and to creme in the homozygous form (c(cr)c(cr)). the dominant d gene is also a dilution gene and is associated with primitive markings (dorsal and zebra stripes)."

North, Ed. Breeding for Color. 1992. Introduction.

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