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Yep! I remember doing this on the HP 48G's in the mid-90's: https://www.hpcalc.org/hp48/graphics/grayscale/

I only ever got four colors to display reliably on my HP48, mostly through naive bank switching/mapping of GROB data.

I remember those! PSA: there's a HP48G emulator for iphone, I use it almost every day...