Wow, that's interesting.
It may be many things; KO/bootleg, custom or perhaps a one-in-a-million variant. Could you please tell us:
- What the stamp says (on his back, something like TAKARA CO LTD JAPAN 1980 1984 with some kanji symbols),
- Where you acquired the toy, and
- If there is any part of him that doesn't match a normal Sludge other than the colour.
Also, have a close look at that black paint. Is any red showing through? Is it sticky, crumbly or the wrong type of paint for the application? Has it chipped off, or does it show signs of age consistent with the age of the toy?
Have a feel of the toy itself. Is it light, loose, or of poor quality? Does it have any stickers?
These things will help determine exactly what it is.