Hello!
I'm pretty new to Krita and I have run into a bit of a problem. I finished a sketch but then, later on, realised that I didn't like the colour of the sketch and that I would like to change it. Is there any way to do that? (other than having to redraw the whole sketch)
Any help is very much appreciated:)

Whatever you try and whatever you do, make a backup copy of your original sketch .kra file, obviously.

The easiest and most flexible way I can think of would be to use your sketch layer to make a transparency mask for a 'colour layer' that has been flood filled in the colour of your choice. This has the advantage that you could do local manual adjustment of colour by painting on the colour layer, or flood fill it with a new colour.

Assuming your sketch is on a transparent layer, you'd need to put a flood filled white layer underneath it then merge it down onto the white layer to give a single 'sketch on white' layer, then invert that to give you a 'sketch on black' layer that you'd use as the transparency mask.
Then, you could use the Levels filter on the transparency mask to adjust brightness and contrast of the new colour sketch.

When you're happy with the end result, do Flatten Layer on it to get your sketch in the new colour on a transparent background.

If your original sketch has lots of subtle texture and opacity variation, you might lose a bit of the detail by this method but give it a try.