I used this: a Red Posca PC-1M Paint Pen, of course you could use any red pen but i chose this as its matches well, its a nice strong red. Its also a water based paint pen, so its permanent on porous stuff i.e the stitching. but wipes off when dry on the leather. I just went real careful and took my time with it anyway so only 1 bit went on the leather and it wiped off once dry. These pens are about £2 on eBay or art shops sell them.
Took about 20mins to do.
Here's before:
After:
Obviously I don't know how long it will last or that as I've only done it today, pretty sure it will hold up well though.
Hope this helps people!