There is a single gray scale common to all the views, axial, coronal, sagital and MIP.
You may move the dual slider like an ordinary scroll bar, but you may also change its width by grabbing either end and dragging it. Notice in the above image how the cursor has changed into a dual arrow when it has been placed over one of its ends. In this case shortening the slider will increase the contrast while lengthening it will reduce the contrast. Grabbing the slider in the middle will change the level, leaving the contrast fixed. The numbers in the boxes indicate that the current width is 10 and the level is 5.
To reset the gray scale to standard values, right click on the image and use the pop up menu.