If it darkens, this is an absorbent leather suite.
It is a degreasing job for sure!
What you see may be just the top 15% the remaining may have penetrated into the leather structure.
The finishing may have already been rubbed off by those hair gels.
Depending on how long the hair gel had contaminated the leather structure.
The ageing gel may give a yellowing effect if it has hydrolyzed or oxidized.
So be prepared for a restoration job.
Here the restoration means to both strata of the leather (structure & finishes).
1st strata is the leather structure itself - to use the latest technology that can degrease “from” the structure rather than “off” the surface of the leather only.
The leather structure is to be further hydrate and fatliquor to restore the leather integrity - flexibility, softness and strength.
2nd strata is the leather finishes to be restored for its aesthetic sight, feel and scent.
Post a picture, the severity of the darkening and the finish-type will determine the product system for this specific problem.
Roger Koh
Leather Doctor®