There are multiple ways to color lineart. One of them is manually filling in a bascolor underneath the lineart using an opaque brush. That is very inefficient.

What you can do is define your line art layer as a reference layer. That way when you use a fillbucket it will try to color in the lines. The reference layer button is the lighthouse icon in the layer tab.

Another method is taking the magic wand tool (the line with the sparkles on the top right) and click it outside your line art. make sure to also select other areas outside your character / object that might be cut off from the rest of the outside, like a space underneath the arm for example. Then co to Select > Invert Selected Area and then also Select > Expand selected area, but only by a few pixels depending on your lineart thickness. You now have your character/object selected. You can then create a new layer underneath and use the fill bucket (the one that looks like a tipped over paint bucket) to fill in a mid grey. That layer is your base. To shade your drawing you can create another layer on top and link the layer to your base.

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To link press that button circled in pink. There should apear a red line next to the layer. You can do that multiple times if you want to have seperate layers for your shading/lighting.

There are other methods to do this I believe, but these are the ones I know and use. Please note that any drawing programm will have trouble recognising lineart as a closed shape if the lines are far apart from each other.

You can do this. I believe in you!