The Amiga 500 had high res graphics (or high color graphics … but not on the same scanline), multitasking, 15 bit sound (with a lot of work - the hardware had 4 channels of 8 bit DACs but a 6-bit volume, so …)
In 1985, and with 512K of RAM. It was very usable for work.
For OCS/ECS hardware 2bit HiRes - 640x256 or 640x200 depending on region - was default resolution for OS, and you could add interlacing or up color depth to 3 and 4 bit at cost of response lag; starting with OS2.0 the resolution setting was basically limited by chip memory and what your output device could actually display. I got my 1200 to display crisp 1440x550 on my LCD by just sliding screen parameters to max on default display driver.
Games used either 320h or 640h resolutions, 4 bit or fake 5 bit known as HalfBrite, because it was basically 4 bit with the other 16 colors being same but half brightness. The fabled 12-bit HAM mode was also used, even in some games, even for interactive content, but it wasn't too often.
Sorry I'm not exactly sure what you're saying. I know very well how it works as I write a lot of demos and games (still today) for mode 13h (see https://www.pouet.net/groups.php?which=1217&order=release) and I can program the VGA DAC palette in my sleep. Were you referring to the fact that you write 8-bits to the palette registers? That's true, you do, but only 6-bits is actually used so it effectively wraps around at 64. There are 6-bits per colour component which as you pointed out is 18-bits colour depth.
Btw I was a teenager when those Denthor trainers came out and I read them all, I loved them! They taught me a lot!
In 1985, and with 512K of RAM. It was very usable for work.