And now the real trick ... Curves!
Go to Colors || Adjust || Curves, and in the "Curves" dialog box, set Channel = RGB.
On the Input / Output grid we will be working with four nodes. If you have less than four nodes, place your mouse pointer on top of the curve line until you see the +ADD sign and click to add a new node; if you have more than four nodes, click on the extra node and drag it off the grid.
Now that you have four nodes to work with, you will be clicking on each one of them (to make it active) and on the Input and Output boxes you will be entering the settings as follows:
Node #1 (bottom left corner of the grid):
Input = 0
Output = 0
Node #2 (1st column, near the top):
Input = 48
Output = 242
Node #3 (3rd column):
Input = 164
Output = 192
Node #4 (top right corner of the grid):
Input = 255
Output = 255
Click on OK to apply the adjustment. If you wish, you can go to Edit || Repeat Adjust Curves to apply it once again.