Kris, these shots don't show you how to do it, but what you can achieve.
This one below looked tricky at first, but turned out to be one of the easiest I've done.
It is also recommended to get in good with all your local carpet manufacturers, so you can access yarn from the same carpet (you're repairing) including same dye lot.
This one had three colours, all with some missing.
Next the black yarn was sewn in
Next the fawn yarn. And this one really gives the carpet it's pattern. So I say this carpet was easy because all I had to do was get this colour placement spot on, and the rest of the carpet (pattern) looks good. (Please excuse the shot. The camera used for these pics has been burnt, destroyed and buried!!)
And lastly the blue was sewn in
You can probably notice a slight lightening in the centre of the last pic, I lightly dust most RE TUFTS (ok, re burling :roll: ) with chalk or plaster board. It looks a bit too light, but standing up, the slightly darker colour (of the new yarn sewn in) could no longer be seen, nor could this 'lighter' affect.
M