This tool is accessible via main menu > Objects > Color in selection / transformation mode.
Tune Colors allows to adjust colors of all selected objects, or even whole design at once. Tune Colors command opens window with Brightness, Contrast, Gamma, Saturation and color balance controls (Cyan-Red, Magenta-Green, Yellow-Blue). These controls work with color of vector objects and stitches (thread), not with the image colors.
Left: original colors before tunning. Right: brightness increased for all objects at once.