Markers aren't ideal to color hair. The ink can easily stain the pony. For lighter hair you can use Rit dye to color it. Turning dark hair lighter can be done by sunfading, but not all colors respond the same way and sometimes it will only bleach out one component of the color so you're left with something you don't expect.
I suggest getting some genuine supplies
A little bit of hair to practice with first would be good. You can look into different rehair methods and see what you'd like to try. I started with the needle and thread method, but now I use a rehair tool. If you want to practice with dye, get some white hair and try the powders, they're cheap