Art If This Is Considered As An Art Block, Please Help...

Hello, and I would like to seek for help on improving my colouring and shading style. The picture below is drew via FireAlpaca. When I start colouring, I soon found out that the colour I used overlaps my lineart. I did arrange the colouring slide below the lineart slide by the way.

Next, I also had problems on shading. As you may see, I didn't add any kind of shading on the picture below. This is because I had some tricky issues with it. For example, when I try to shade I used the overlay blending and picked my desired colour. Suddenly, it turns out the shade colour is not the colour nor shading that I wanted to be. I try to add warm colours, blue turns green. The exact opposite occurs when I used cool colours as the shading colour.

If you guys have any guides or tips on improvise colouring/shading. Thanks for your help. As a notice, I am not forcing you to help me. Help if you able too!

 

I think the problem is your overlaying.
I use paint tool sai, though I am sure it is somewhat similar to firealpaca. What I would normally do first, is sketch then line art (note that they are both on different layers).
Cover up the sketch lines, and add a new layer under your line art. Add base colours to your image. Depending on what type of shading you do, you may use different layer setups and brushes. For cell shading, I would normally select areas that I want to shade using the select brush then fill them in with my desired colour ctrl+f. I recommend paint tool sai, as I'm not too familiar with firealpaca.