deriv LSK ETT STT aSTA ALPH OLDHOMEPAGE NEWHOMEPAGE
A denominative is a verb root derived from a noun.
By "derived", do you mean that the noun existed earlier and the verb was later derived from it, just like in English the noun "brush" is attested in the thirteen hundreds but the verb "to brush" in the sense of "clean with a brush" is not attested before the 1400s?
No, in Sanskrit culture a verb root is said to be "derived" from a noun when the grammarians did not put it in the /dhAtupATha, but instead they explain it by adding a root-making affix after a noun. Example. The word **putrIyati पुत्रीयति is clearly a verb, so it must have some root. The grammarians say that the root of this verb is putrIya पुत्रीय, and that this root is formed by attaching after the noun **putram पुत्रम् the root-making affix /kyac.
Is the verb varNayati वर्णयति derived from the noun **varNa- वर्णॱ?
In the Western sense of the verb "derived", it is, because research strongly suggests that varNayati वर्णयति appeared historically after varNa वर्ण.
In the /pANinIya grammar system, however, varNayati वर्णयति is not a denominative verb. It is a /curAdi verb like **corayati चोरयति, because old grammarians happened to include varN वर्ण् in the /dhAtupATha.
Why did old grammarians do that?
Because deriving the noun **varNa- वर्णॱ from that verb root adding a /kRt affix is less trouble than deriving the verb from the noun adding a root-making affix.
Were they unconcerned about historical accuracy?
Yes, completely. Historical reconstruction's a Western cultural thing. They just wanted to make the grammar system work.