Using the empirical pseudopotential method, the electronic and optical properties of zigzag and armchair colloidal ZnO nanoribbons have been studied focusing on their thickness and width dependence. Our results indicate that contrary to what has been theoretically predicted for epitaxially grown nanoribbons, both colloidal types exhibit semiconducting behavior. Furthermore the absorption spectra for both nanoribbons are mainly dominated by y-polarized transitions while for the case of zigzag nanoribbons there is a higher energy x-polarized peak. The increase of confinement along the y-direction in zigzag nanoribbons leads to a suppression of the y-polarized peak, leaving back a purely x-polarized optical gap.