How Do You Shade Colors in Krita?

First, you need to determine where the light source is coming from. Then you will add the shadow on the opposite side of the shape or object. 

For example, I have a circle, and the light source comes from the top right. So I need a shadow on the left side of the circle and the ground. I have drawn an oval on the ground for the shadow.

Next, you need to section off the area where you want to add the shading using a new layer. You can use any brush to draw the line. Increase the Sample Count and the Delay in the Tool Options docker to create a smooth line. Once your brush is ready, draw the line across the shape.

Now you can select the Fill Tool from the Toolbar or press F to activate the tool.

Set the foreground color to black in the top settings bar.

To correctly fill the shaded area, there are a few necessary settings to change in the Tool Options docker. First, set the Spread to 0%, then set the Growth to 2 px. The Growth setting prevents the fill from leaving an unfilled gap next to the outline.

Now, you can add the black fill to the area by clicking inside the sectioned-off area. This will fill the area with black.

Once the area is filled, you can erase the overlapping lines using the Eraser brush.

Now, if you want a bold shadow, as some animations need a completely black shadow, you can leave it as it is. However, if you want a more subtle shading, you can lower the opacity of the shade layer to blend it in better. 

To change the opacity, go to the Layers docker and drag the Opacity slider to the left to decrease it.

Once you drag the slider down as much as you need, you will have a shaded area on your object.

You can now repeat the steps to add more shading to your project as needed. I repeated the steps to add shading on the ground for my image.