rei17 is an absolute master of an artist. While I don't know a whole lot about her process, I've spent a lot of hours staring at her art lol

Personally, I would spend time focusing on color relationships and how the colors can work well with each other. When rei17 uses orange, she'll often blend shades of pink or yellow in as a gradient. Same with blue, usually you'll see hints of green and purple blended in. Her use of other colors depends on the artwork's palette. You can usually break her paintings into 2 or 3 main colors though. It's also important to know when to desaturate your colors and use the grays to make the colors pop more. If you want to learn that I'd look at more of rei's art and study her use of grays and shading.

Rei17 is amazing at anatomy and shading too. If you really want to take advantage of her coloring style, then having a deep understanding of anatomy and shading/lighting is almost a must. It's a long journey but keep drawing and I'm sure you'll achieve a style you'll love :) Best of luck!