blonda

English and French both make you choose a spelling before you can use this word: blond for a man, blonde for a woman, and a small etiquette problem every time. Esperanto deletes the question. Adjectives agree with number and case and never with gender, so blonda is blonda for everybody, and the only endings it will ever take are -j and -n.

The word is about hair. You can stretch it to a beard, to a field of corn or to a glass of beer and every reader will follow you, but describing a person as blonda means their hair, exactly as in English. For the person rather than the property, -ul- does the job: a blondulo is a blond, a blondulino a blonde, and now the gender is where Esperanto normally keeps it - in the noun, marked once, optional.

The neighbouring colours are worth learning in one go, because they are the ones a description actually needs: bruna brown, nigra black, ruĝa red, griza grey. Note that a redhead is ruĝhara rather than ruĝa, and that hela (light) and malhela (dark) will get you through most of the rest.