For negative only, the best way is the popup menu and choose "Neg Only". Please note that this menu applies to the threshold value ("Thr" sub-brick). That is, if you have "Neg Only" chosen, then only voxels with threshold values that are negative and below the minus of the slider value will get color.

The color they get is from the "Olay" sub-brick. In some cases, that might be positive. (It will not be if the OLay and Thr volumes are beta coefficient and corresponding t-statistic.)

The "Pos" button turns off displaying colors for negative OLay sub-brick values. That is somewhat different that turning off colors for negative Thr sub-brick values, as the "Pos Only" popup menu item does.