Hi Zhomo,
I used a pad on the bolster dabbing where neccessary only and a toothpick on the seams, cracks and airholes on the main seat.
I then diluted the dye and blended to avoid any colour mismatch.
If I am critical, I can still just see a slight variation in colour on the seat back bolster and next time may colour the whole top side bolster lightly as the seam would hide any colour difference.
Maybe 4RE Leather could advise on this.
I am being really picky on this though as you really have to look closely to see the variation, my wife thinks I am being OCD!
The biggest challenge now is to get her to sit on the base of the seat and twist in rather than sliding down the bolster...what chance!!!
I would leave it. the variation is the fact that the untreated part still has some human contaminants in it like body sweat transfer (leathers greatest enemy)
Plus the sun will cause its own colour difference.
The only way to eliminate is to do the whole seat in one go.
Once you put some light conditioner on in about 4-6 weeks it will all blend in and look great.
Not that it doesn't already