change hex color to ligher or darker

I need some kind of function that can i change a hex color to lighter or darker.

Example:
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strColor = LightenColor("#000000", 50)
strColor2 = LightenColor("#FFFFFF", 25)

Function LightenColor(strHex, strValue)
      'strHex holds the color
      'strValue holds how much to change
      'Code to lighten color
End Function

Function DarkenColor(strHex, strValue)
      'strHex holds the color
      'strValue holds how much to change
      'Code to lighten color
End Function
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Thanks in advanced.