Sorry to necrothread but I just uploaded ColorDesk to Macintosh Garden after realizing I couldn't find it online anywhere.
I prefer ColorDesk to DeskPicture because it loads at INIT time, so cosmetically gives a nicer boot experience - your color background is already there behind the Finder when it first appears (DeskPicture loads later after a visible delay).
As I recall it's compatible with System 6 and maybe even earlier ... and I can confirm firsthand it works with 7.5.5 and 8.5.
https://macintoshgarden.org/apps/colordesk