Distinguish clearly between complete dominance and co dominance.  

In complete dominance, the dominant gene/allele completely masks the effect of the recessive gene/allele whereas, in co-dominance neither gene/allele in the pair is dominant

Or

In complete dominance, only one allele in the heterozygous state is expressed in the phenotype. In codominance, both alleles in the genotype are expressed in the phenotype.