It is going to be a lot easier to dye an unfinished leather than a finished leather for sure. This took me about 2 minutes


All leather will naturally oxidize in the sun and darken over time, more so when they are oiled. It can help to put your stuff in the sunlight after you oil it (not too much heat though). And as someone mentioned warm oil will help open the pores in the leather. Beware or over-oiling though and avoid using neatsfoot compound as it will rot the stitching. 100% neatsfoot is ok. 

"It's really quite amazing what a horse will do for you, if he only understands what you want. 
And it it's also quite amazing what a horse will do to you if he doesn't." ~ Bill Dorrance