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no date? edit
when were these developed? when did they become used in artists' colours? — Preceding unsigned comment added by 174.95.219.223 (talk) 20:19, 21 July 2019 (UTC)
R-group edit
I know what an R-group is, but I can't be sure how to interpret what the writer meant by it in the context of this article. Please be clear in your writing. — Preceding unsigned comment added by 77.61.180.106 (talk) 10:08, 6 May 2021 (UTC)