Changing backcolor or brightness

Question

Ok, now that I have the color chosen from the ColorDialog and it has been applied to the forms, how do I get a color that is slightly darker (if a lighter color was chosen) or slightly lighter (if a darker color was chosen) for the various backcolors? i.e. if somebody chose blue, I want to automatically change the various backcolors to light blue so that the textboxes, listboxes, etc stand out more. This is a tall order and I greatly appreciate any help on this!  Or better yet, just change the brightness, if that's possible?!

Here's the code for the dialog:

Dim mycol

        cdlg.ShowDialog()

        mycol = cdlg.Color

        Me.BackColor = mycol
        FontStylesAndSizes.BackColor = mycol