NOTE: If you're going to review this, PLEASE refrain yourself from telling me that I should make this piece better this by adding some shades, lighting, 'actual' background, etc. I wanted to draw in a simple design instead of a more detailed design like most people expected me to do more often. Review the piece for what it is, DON'T tell me to fix things that aren't even broken in the first place.
First off, take G3no's advice...
What I like to do (in photoshop) is create a layer with a solid black on top of the image and set the blend mode to 'color': https://i.imgur.com/RylXj7T.jpg
Since there's such little detail I'm forced to pay special attention to 2 things: values & colors. The colors are good, but if you take them away you can see that her hair nearly blends in with the background. You want ALL elements of the picture to be strong independent of one another. I would lighten the background or something to create more of a difference between the values there. The reason that shadows & lighting help is they make it so this is less of an issue because they create those value differences automatically. Most people know this intuitively but they don't know enough about art to put it into words.
In terms of simplicity, It's it's own style, shading is optional in the terms of minimal art. Your matching of various colors and shapes to give the illusion of depth is what matters, and you did good :]
i feel bad bringing up stuff that is irrelevant to the piece but this caught my eye. watch out when telling the internet what to do , it can be ruthless at times as they will still critique anyways and milk your irritation. it's up to you to rationally filter the genuinely (emphasis on genuinely) constructive advice from the spam. just never allow for others to see that they got to you. it's bad news.
you got front paged, be happy. it's indeed very cute
I like it a lot ,but the way you're already very defensive about it feels bad.