Remember that unfinished leather derived its color from the tanning agent itself and there is no color add-on to it thereafter. Any color put on to them may make the leather looks fake or unreal. Not correct or appropriate but yet it may seems to be the last option out for cosmetic purposes, need to seek out customer’s approval.
Technically it is a refinishing, not possible to touch-up to match the unfinished leathers and again there are the stitching threads for consideration as well.
d’Tarnish-1.3 is also a possibilities for lightening up the color to try.