Multipaint 2018b by Tero Heikkinen - http://multipaint.kameli.net/

With Multipaint, you can draw pictures with the color limitations of some typical 8-bit computer platforms. The screen formats supported are Commodore 64 high resolution, Commodore 64 multicolor, Commodore Plus/4 Hires, Commodore Plus/4 multicolor, ZX Spectrum, MSX 1 and Amstrad CPC0.

Requires Java, but just having the ability to export your drawing directly to a .prg file that will run on a C64 makes it worth the sacrifice.