I think you can get it very close, but unless it's white, it will still be somewhat visible (at least to you).
If you take some of the colored fondant and mix in just enough water to make it piping con to consistency, you can pipe it into the seam. Even it out, let it dry (it'll sink into the seam a bit), and then do it until it's flush, carefully scraping it off of the surface of the sides each time you apply.
Another option is to tint royal just a tad darker and pipe that into the seam. It dries lighter, so that's why it needs to be a bit darker to start. Again, carefully add and scrape until it's level. The issue with this method is that if the cake shifts or settles, the royal can crack.
HTH
Rae
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