I have access to the OED.
The OED notes that “early quots. are in the speech of American Indians”, but in fact they are in the reported speech of American Indians - reported by whitefellas. Plus, either the American Indians spoke in their own language, in which case the report involves translation, or the American Indians spoke in English or French, in which case might they have used the terms which they understood their (white) interlocutors to use?
The earliest cite is from c. 1769:
[QUOTE=OED]
tr. Mosquito in Papers Sir W. Johnson (1931) VII. 137, I shall be pleased to have you come to speak to me yourself if you pity our women and our children; and, if any redskins [Fr. quelques peaux Rouges] do you harm, I shall be able to look out for you even at the peril of my life.
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Then there’s a cite from 1812:
[QUOTE=OED]
French Crow in J. C. A. Stagg et al. Papers J. Madison Presidential Ser. (2004) V. 182, I am a red-skin, but what I say is the truth.
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And that looks as though redskin might have been pejorative by then.
From 1815:
[QUOTE=OED]
Black Thunder in Niles’ Weekly Reg. 14 Oct. 113, I turn to all, red skins and white skins, and challenge an accusation against me.
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And from 1823:
[QUOTE=OED]
J. F. Cooper Pioneers ii. xvii. 256 There will soon be no red-skin in the country.
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What all the early cites have in common is that they point to tension and mistrust between American Indians and while settlers. In that context, “redskin” might have been almost inherently pejorative in the mouths of whites, and if American Indians adopted the term and used it in discourse with whites, that might have been deliberately confrontational; challenging language, rather than an acceptance of the neutrality of the term.
On the other hand, the OED point out that the term comes into English from French, and that it came into French apparently as a transliteration of an Illinois term meaning “person with red skin”. Similarly, the use of the term in the 1815 quote is said to correspond to a Meskawi term (Meskawi being the vernacular of the speaker in that quote) which meant “person with brown skin”.
In short, the OED does suggest that the French, and then the English, acquired “redskin” by transliteration from one or more American Indian languages. The OED doesn’t suggest that this had anything to do with battle paint or body decoration.
One other point: Although the term has cites in English going back to at least 1769, the OED notes that it was popularised by J Cooper Fenimore, i.e. well into the nineteenth century, by which time negative attitudes to the American Indians were deeply entrenched. So while it may have originated as a neutral term in the mouths of American Indians, it may have been received into English as a pejorative.
The OED notes that it’s now “somewhat dated and freq. considered offensive”, and that highlights the point that whether the term is offensive or not is not determined by its origins, but by whether people are offended by it.