Running one of the latest NzTiVo or OzTiVo images on a UK Thompson TiVo should give you colour output by default.
It may be possible your SCART to S-Video cable is only transferring the Luminance (brightness) signal and not the Chrominance (colour) signal - both which make up a full colour picture in the S-Video format.
Easiest way to check is to try another cable. I recommend to get one of those SCART adaptor plugs that has a SCART plug on one side and at least a single RCA video connector (or one with both the RCA and S-Video connector) on the other so you can 100% rule out a problem with your current hookup.
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