Indeed. I did copper too, in DOS x86 assembly. Some programs used it to practical effect: you can exceed 256 colors in an 8-bit framebuffer by swapping palette values mid-screen or mid-scanline.

In fact, every Atari 2600 game is a copper effect. The 2600's graphics chip is one-dimensional, working with only one scanline at a time. To display a picture, the software must run in lockstep as the electron beam traces down the screen, changing sprite bitmaps and colors and positions each scanline as appropriate. In other words, the 2600 literally uses the phosphor on the physical TV screen as the frame buffer. No surprise that this was tricky to emulate, and why 2600 emulators took longer to reach usable compatibility levels than emulators for the later more powerful Nintendo systems.