I've seen the effect that the grouting pen created and wasn't keen on it but that was many years ago so they may have improved on the quality/effect.
Would you not try cleaning the grouting first. Its a long and hard job but I always found the good ol' toothbrush with Cif cleaner did the best job.
Presumably you are now applying the "Prevention is better than cure" method and have stopped smoking?
Grout pens don't leave the neatest or nicest finish.
Cif, diluted bleach or a steam cleaner should work.
Bleach on a toothbrush - it's what I use for mould in the shower...
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