When taking separate left/right images for a stereo-pair, the exposures may not be matched for a variety of reasons.
SPM's 'Auto color adjustment' uses histogram-matching to try to adjust the color/brightness of the right image to the left 'reference' image.
If necessary, press 'X' to swap-over the images so that the correctly-exposed image is on the left, as in the above example.
Simply click the toolbar button, press Ctrl+K (for 'Kolor' adjust) or choose 'Adjust/Auto color adjustment' from the menu.
If you are not satisfied with the result, you may choose 'Edit/Undo' from the menu.