The term sgraffito comes from the Italian word sgraffire which means (literally) “to scratch”. The technique involves scratching through a layer of still-wet paint to reveal what’s underneath, whether this is a dried layer of paint or the white canvas/paper.
Any object that will scratch a line into paint can be used for sgraffito. The ‘wrong end’ of a brush is perfect. Other possibilities include a fingernail, piece of card, sharp point of a painting knife, a comb, spoon, fork, and a hardened paintbrush.
Don’t limit yourself to scratching a thin line; broad sgraffito with, for example the edge of a credit card, can also be very effective. If you’re using something sharp, such as a knife, you need to be careful you don’t accidentally cut the support.
And don’t limit yourself to using the technique with just two colours. Once your top layer has dried, you can apply another colour on top and scratch through this. Or you could apply a range of colours in your bottom layers so different colours show through in different parts.