Hi, I'm obviously new to not only krita but digital art in general and I'm searching for a function I've seen other people use, but I can't find it with a google search, possibly because I can only describe it, I don't know what it's actually called.
It's when you have for example a black and white object on one layer and then create another layer on top for coloring but you'll only color on where the black and white is on the previous layer.
What is this function, where can I find it, is there a shortcut?
I hope I explained it well enough, I'd really appreciate some help!
Thank you!!!

What you describe is fairly simple to do. I assume that the object you want to colour is the only object in a transparent layer and that it's not important that the object is black and white. It could be green and blue or whatever.

The simple way is to use 'alpha lock' which will limit any painting/colouring to parts of the image that already have content. This is destructive in that you'd be painting over the original image, i.e. not using a new layer. So it would be a good idea to work on a copy of the original layer.

At the far right of the layer, in the layers docker, there is a small icon that looks like grey squares. If you click that icon, a dark 'lock' symbol will be shown on top of it. This indicates that alpha lock is operating on that layer and that paint can only be applied to areas that already have paint on them. It's easier to do it and play with it than to read about it. Playing with Krita is a great way of learning how to use it, in combination with reading the documentation and the posts on this wonderful forum

https://docs.krita.org/Main_Page

Another way is a more sophisticated method which uses additional layers and is not destructive to the original layer and gives more control and flexibility of effects. This uses 'alpha inheritance' and is described in the manual:

https://docs.krita.org/Clipping_Masks_a ... nheritance

At first sight, this looks complicated but it's not all that bad once you play with it and get used to it. Everybody gets totally confused by alpha inheritance when they first try to use it so don't let that worry you. Alpha inheritance will only allow paint to be applied where the layers below have paint. The important thing to remember is that alpha inheritance will cause the layer it's set on to be affected by every layer underneath it. This is why it's a good idea to group the layers you intend to use alpha inheritance on because alpha inheritance doesn't come in from outside the layer group, unless alpha pass through has been set on the group. Alpha inheritance is difficult to understand until you've done some simple experiments to get used to it. The advantage of using alpha inheritance is that you can have separate layers for colouring, texturing, highlighting etc.

Thank you so much for the detailed answer and links, now ill be able to find it, play with it and learn!!