I sometimes hear discussions about whether “loan” is a verb.

Problem:
“Loan” in American English is a verb, but it has a specific meaning.

Explanation:
British English tends to avoid using “loan” as a verb, and British critics many years ago identified the use of “loan” as a verb as an Americanism.

Even though this website focuses on American English and not British English, there is a distinction to be made between “loan” and “lend” as verbs.

Some Americans use the verb “loan” as if it were completely synonymous with the verb “lend”, but this is a mistake.

The verb “loan” specifically means to make a loan of, and the noun “loan” specifically applies to physical transactions.

So it is correct in American English to ask “Can you loan him your car?”, although I prefer “Can you lend him your car?”

But it is incorrect in both British English and American English to say, “The colors loan the painting a sense of lightness.”

Solution:
If your audience is British, then never use “loan” as a verb. If your audience is American, then use “loan” as a verb only when referring to the lending of goods or money but never when referring to figurative transactions. The simplest solution is to avoid all use of “loan” as a verb.