Why does Lenina hate the color khaki? Why is she glad she is not a Gamma

Lenina hates the color khaki because it belongs to the Delta, one of the lower caste, made up of individuals of low intelligence who work in factories or as drivers of vehicles. As she is of a superior caste, she considers everything assimilable to the lower castes as ugly or unimportant.

In line with this thought, Lenina expresses her joy at not belonging to the Gamma, who, again, was an inferior caste to her own, the Beta.