All it takes is practice. I started off with basic recolors, then pokemon based recolors. After you get the hang of that, playing around with sprite parts is pretty easy. Are you using Photoshop? Also I recommend these two sites for pokemon based trainer recolors:Dude, youre sprites are like one of the best i've ever see. Theyre clean and they look amazing. Your sprite art has inspired me to make sprite. So thank you, I thank you from the bottom of my heart.
http://html-color-codes.info/colors-from-image/ great for getting specific colors from an image in html color code
If you need to convert the html to rgb, use this site:
https://www.easycalculation.com/color-coder.php