Just goes to show that the border between nouns and adjectives is fuzzy.

Quite fuzzy. The Roman and Greek grammarians had a single lexical category (new-fangled speak for part of speech) called nomen, nominis 'noun', literally 'name'. Nomina were furthered subdivided into nomina substantiva 'names which stand for' and nomina adjectiva 'names which increase' or 'hurled at names'. The Greek grammarians came up with this distinction and the Romans basically copied it and translated the terms. The analysis is based on the fact that there is no way to determine (by form or morphology) if a nomen is an noun or an adjective: the case endings used are identical, and adjectives could be used by themselves in a Latin sentence in the same way as a substantive. Adjectives weren't spun off into their own category until late in the 18th century.

popsicle toes

The word toes is definitely being modified by popsicle, but whatever the lexical category of popsicle is (and I go for noun) it's not the same as the category for adjectives. I base this on what slots the word can fill in sentence templates: e.g., the following is not grammatical for me: "my toes are very popsicle". To me, popsicle toes seems uncontroversially a compound noun of the form: baseball, babysitter, etc. Your analysis may vary.

Ceci n'est pas un seing.