As in this recent example I came across:
"I stick out like a pale thumb in a sea of tan blonde hipsters".
I find this interesting, because to me tan is either a verb or a noun, the standard adjective being tanned. For example you can go and tan (verb) in the sun, and you might get a tan (noun), but as a result you will be tanned (adjective). This fits with a rather general pattern of forming adjectives from past participles (pp) of verbs. (Examples: to grow - I have grown (pp), thus I am a grown (adj.) man; to pop - The internet has broken (pp), thus we have a broken (adj.) internet; to close - the shop has closed (pp), thus we have the closed (adj.) shop.)
However there are examples of adjectives being the same as the uninflected (unaltered) form of the verb. For example, if you clean yourself, you become clean, you don’t need to follow the pattern and say that you’re now cleaned. So since words change word class all the time (here I think we’re going from verb to adjective), and since tan as an adjective is not morphologically* inconsistent with existing patterns in English - clean being one such precedent used in the same way that tan is now being used - I’ve got no huge problem with it, I just find it very interesting.
*Morphology - regarding how existing words are manipulated, mainly through suffixes and prefixes, for example the ‘-ed’ suffix to make a past tense; ‘-s’ to make a plural; ‘un-’ to make a negative or ‘-ly’ to make an adjective into an adverb.