Thanks with the lineart thing, it will definitely help.I use "Normal" colour layers for the base colours, then there's two different ways to add light and shade.
If you create a normal colour layer above the base colour, then you clip it and set it to "Multiple", and use the same colour as you did the base, it'll automatically darken it down, then you can use that for basic shade.
If you do the same thing, but set it to "Add", it should create light.
Sometimes that doesn't work though (okay, quite a bit of the time - especially if you're using fully-saturated colours, but still...); I've found that certain colours look weird with those kinds of effects (either that or the effects don't work).
So, in that case, you'd have to choose your own shading colours. Which is pretty easy, because all you do is shift hue slightly, add in more saturation, then darken it up a bit, and voila.
View attachment 1421511
There's my usual kind of set up.
I always, always put the light above the shade. Because, usually, if you look at light in real life, it cuts through shadows and shade.
About your lineart problem, keep your lineart in a separate layer at the top of the list. That should keep it above the colours, so you don't have to keep drawing it in. :3
If you meant about colouring and having to trace the colour's edge to get rid of white patches: Expand your paint bucket. I'm not sure if you're using the computer version or the mobile version, but somewhere in both of those you have the opinion to make the bucket fill in an extra pixel - so find that and it'll fill properly.
Honestly, poke around with your tools a bit more. Try different layer types out, different effects, all the different things. You'll get to know what stuff does what and you'll get to know where stuff is. :3
(I'm glad I can be of help. :3 I'm not the best artist, by far, but I know some tips and tricks that might help. XD)