Custom cursors should not be necessary. For brush or pencil, a dot in a circle works the best; it is a precise, simple and clean solution. In a paint bucket cursor the user needs to realise that the tip of the paint is the actual cursor (something I learnt with trial and error). The dot in a circle can be scaled to have multiple dimensions such as good feedback in terms of current stroke width, brush pressure (higher pressure means darker and thicker strokes). If a custom cursor can provide advantages over these only then it will be worth testing with primary users.