I used the instance node to point-instance cubes onto a sphere.

The sphere has random colors on its points. I want the instanced cubes to pick up this color.

I know I have to use the instancepoint() function but wherever I put it nothing happens.

Where does the instancepoint() function go?

use it on object which you are instancing
or its material

it will give you point number which the object is instanced onto

if you need the color then, you will need to use point() expression to get the point color from the instance points based on instancepoint()
you can also use it in material overrides as in attached file