Need advice on a color that matches this color theory
This is based simply on classic color theory:

You're currently using two colors, and want to add a third. So, using color theory would involve choosing the third color based on the relationship of the first two to one another.

Since the coral orange and dusty purple (based on my screen) are about 1/4 of the "wheel" away from one another, there's really two color schemes that fit:


Light, lime green fits in with a triadic color scheme.
A rose pink could fit in quite nicely with an analogous scheme. 


Triadic color schemes have 3 colors that split the color wheel into equal thirds. Closely related is the split complementary theme, where two of the colors are adjacent to the complement of another. In my example, you'd be using a blend of both. Your first two colors aren't so close together that the complement of the lime green, rose pink, is actually adjacent to them. That's why it's more of a triadic color scheme, despite not being a perfectly even split around the color wheel.

The analogous scheme is using three colors that are near each other, the "outer" two (on the wheel) about the same distance from the center color. That's how I arrived at my rose pink suggestion.

It's important to remember that there's room to play, and color theory can be used to develop rich palettes with many distinct colors, even if it's only three base colors from the wheel. And the color you pick can affect the mood or tone you're trying to set in your work. Consider the difference it would make shifting the lime green closer to yellow or green. Or making the light green even lighter, or darker, which would add contrast of brightness.

Here's a quick example palette with the four colors discussed (roughly), as generated from the link I provided to Palleton (a simple interactive color wheel with built-in scheme options).