I think it would be interesting if what we saw of Sunstone in "Change Your Mind" was not her default form, but instead what she looks like when her "flame" is on. If this is the case, I believe that when Sunstone is not in danger, her "flame" will be out, and when she's in battle (like in "Change Your Mind), her "flame" will be on. This would fit Sunstone because of Steven's powers being tied to his emotions, along with Ruby's pyrokinetic abilities. Just an idea I wanted to share. Also, real-life sunstones can be darker shades of red, which is represented in my interpretation of Sunstone's appearance when her "flame" is out (on the left image below). I'd love to hear what you think about this theory below!
However, I don't think their skin color would change.
Thanks! However, Sunstone's skin is colored to represent fire (her skin is lighter at the top and darker at the bottom, much like a real-life flame is hotter at the top and cooler at the bottom), and I think it would look a bit strange if the colors didn't change. Also, I didn't add a new color for her skin tone, I used her darkest skin tone to represent that she is "cooled of" when her "flame" is out.