The 2068 was the most mature of the Sinclair line. Not only did it have
color and audio capabilities, but it also had more memory (32k) and expansion
capabilities via a cartridge port and joystick ports. This
machine, too, was Z80 based, although designed around a Z80A. I
acquired one of these rare beasts as of 2001 (from the Austin, Texas Goodwill
Computer Works for $10) and have written some simple Basic programs for it.
It's quite an enjoyable machine to use!
Download a copy of the technical manual in Acrobat (pdf) format here. (Keep in mind that it's nearly 3MB, and so therefore may take some time to download.)