The right one is what they are both supposed to look like. I treated both boots the same, and one turned lighter in color while one stayed almost exactly the same.
I never had cowboy boots. But I wore (and still wear) leather shoes, and wore leather jackets. Eventually, some of them got damaged, due to wear usually, especially the jackets.
I applied the following procedure every time, and every time I was extremely satisfied with the results.
- Clean the dirt / dust / whatever from the area. I used some wet cloth, eventually with a bit of normal soap (soap only if I was not satisfied with the cleaning).
- Wait for the area to dry completely.
- Rub the area with some alcohol, to remove eventual grease.
- Apply some coloring agent, suitable for leather. For me, shoe polish worked like a charm every time. You may need to be creative, if you really want a special shade. Apply it with a shoe brush. Remove the excess and polish, using the polishing shoe brush. Repeat until satisfied.
- Let it "rest" for maybe 1-2 days - if you think it is needed.
- Remove all excess of shoe polish.
- Apply a grease for leather. I was always satisfied with what I call "paraffin oil" - it was cheap and available for purchase at pharmacies. Remove excess.
- Let it rest for at least 1 day to understand the results. If needed, repeat 7.
- Carefully remove any excess of grease. You do not want all your clothes oiled.
- Happily wear your new old boots.
Note: For the "painting" part: I fixed some shoes, and the shoe cream was a totally different shade of brown. To make things worse, the leather was not suede, but still strongly absorbent. For the moment I thought I ruined the shoes, until I decided that the new color was not bad, but only the difference of colors.
Since I was in a hurry, I used different shoes. Later, I applied enough shoe creme on both shoes, until they looked OK again. I was happy afterwards wearing them.
I plan to apply the same treatment to my car's steering wheel (covered with natural leather).