I'm new to Krita so this may be obvious for some people, but I did some lineart for a drawing and I wanted the lines to be a specific color. I used selection to select the lines and tried coloring them, but it just shaded it towards the color. In addition the original line is lighter in some areas so it colors differently there to. I tried creating a new layer to color on, but it still didn't color as wanted. If I go over it multiple time it gets closer to what I want, but that will take a while and I am **** off because this should be something simple to do.

Which version of krita and which operating system are you using?
What colour is/was your original lineart?
Did you make a copy of the lineart layer as a backup before you started trying to change its colour?
For the lineart, did you use a brush that has a 'feathered' edge?
Which selection tool did you use to select the lines?
Did you tick the Antialiasing option for the selection tool? (This is needed for sharp-edged selections.)
If the original line is lighter in some areas, is this variety of line appearance important to you?

There is a fairly simple way to deal with this situation by initially using the Threshold filter to give a pure black sharp-edged line on white background image , assuming that's an acceptable initial result for you, but it would be useful to have a better idea of the situation you're faced with.
If it's important to maintain the original variations in line opacity/shading then there is a way to do that but it's a bit more complicated.

Can you post a link to a full screenshot of your entire canvas and also a link to a zoomed in close up of a particularly difficult part of your lineart?