Considering hybridization in a trait like the color of the flowers of a given plant species (red/yellow) conditioned by a pair of different alleles in relation to complete dominance (red dominant/ yellow recessive), why in the F1 generation is one of the colors missing?

In this monohybridism one of the colors does not appear in the F1 generation because their parental generators are pure, i.e., homozygous, and in F1 all descendants are heterozygous (each parental individual forms only one type of gamete). As only heterozygous genotypes appear and red is dominant over yellow the individuals of the F1 generation will present only red flowers.