Making a font colour darker or lighter

I have a web process that adds words in a certain colour to different images and combines them into a single image.

I want to provide the user with a slider, to indicate making the font colour either lighter with values -1 to -5
or darker with values +1 to +5.

Is there a way to do this with .net c#

So the colour base is not changed, it just becomes lighter or darker shade of that colour
when provided with the values -1 to +5?

Thanks for your help.
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Two options:

built into the .net framework (ControlPaint Class):
external code (free) from Rich Newman (HSL color class):

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stephenwildeAuthor Commented:
Thanks used  for % of darker colur
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