Ok, tis gon be long.
Coloring tips are basically endless, but I'll be basing these off your work. Also, I'll be using my own work as an example, granted I too make mistakes so please bear with it, but I don't want to post other people's work in case they wouldn't like that.
1.
Define your light sources. So far, I noticed you're doing something that is commonly called "pillow shading" in the artist community, aka you just put random shadows next to your lines. It's very clear when an artist has little understanding of light, as shadows don't make sense on their picture and people will sadly notice that. Make sure to define your light source before you shade, then draw accordingly. Think of a lightbulb being next to your character (heck, even draw one next to it, so it helps you remember where your source is) and shade with that in mind. Is it on the left, right, above or below? Or even somewhere else? Maybe there are 2 light sources or more?
For example, in this picture, the light source is on the left side. You can clearly see it especially around his face and hands. Since light is on the left, anything that is on the right gets darker.
2.
Contrast, contrast, contrast. So far your chibis look as if they hardly have any shading due to lack of contrast. The shadows not only are all very light, but they're also all very soft, which makes the pictures look as if they lack definition. I suggest you make harder shadows as well, especially around the places that would be the darkest. But first of all, don't be afraid of using darker colors or else everything comes out like milk. Don't worry though, you're not alone, it's a very common thing beginner artists do (I've done it too!) because they're scared of adding too much contrast not to ruin their picture. Think in 3D, shadows give characters form. Also, don't just shade the base color, add lights too.
Also, if you struggle with contrast, it is not a bad idea to draw your picture in grayscale first and then use layers on overlay/soft light/other mode of your choice to colorize that. When coloring, inexperienced artists tend to focus on the colors themselves and forget about values (shadows) so it's a really helpful technique. And it's nothing to be ashamed of either, many pros use it too!
3.
Colors. As it has been mentioned, you use darker shades of the same color to shade. This in most cases, trust me, doesn't look well. Same goes for shading with black or grey - that's an ultimate sin. I am not sure if you do that, but please never do that.
- Embrace color theory. Learn the basics of it, embrace them, eat them for breakfast. I cannot stress this enough! Once you get even one of the basic rules down, such as what complimentary colors are, you will be able to wing most of your color compositions on a whim. I am not exaggerating. It really is that helpful. Here is a great video that taught me a lot, it's a bit lengthy but worth every second of your time.
https://www.youtube.com/watch?v=Qj1FK8n7WgY
- A few good rules to go by (of course, you don't have to stick to them but they help a lot) is that for example, shadowed areas will be usually less saturated than the bright ones. So remember to tone down your shadows a bit instead of making them very colorful. The other tip that I freaking love and it was really groundbreaking for me when someone told it to me: in nature, under most circumstances, if light is warm toned, shadows will be full of cold tones. And vice versa - if your light is cold toned, then your shadows will be warm toned. I think this has yet to fail me when picking my color palettes.
- You asked what colors do people use for skin. ANY. I mean it - any. It depends on the mood of your picture, and honestly, if you pick the right shade, you can use some wacky combinations. Perhaps the purple you tried using was too saturated and that's why it didn't look right.
Here is some examples of the colors I use to paint skin. First is shaded with mostly brown and blue, the second is mostly purple and reddish tones, and the third uses a totally different palette for the skin overall (ignore the anatomy and such though, that one is from last year.)
Ok, I think this is all I have in mind now, at least before I get my nap. I hope this helps you and I wish you lots of luck with art.