An airbrush is the ultimate tool to create a gradient fading effect with paint on a pair of custom shoes. But a good looking fade can be created with a steady hand and a decent paint brush. Checkout this first tutorial to see how shoe customizer SBTG of RoyaleFam creates a fade on a Nike Swoosh without an airbrush. The final effect is more of a linear fade instead of paint splatter but you achieve the same effect nonetheless. I would also recommend masking off if you do not plan to paint the area around the Swoosh.