As a simple approach, you can just factor the RGB values:
Color c1 = Color.Red;
Color c2 = Color.FromArgb(c1.A,
(int)(c1.R * 0.8), (int)(c1.G * 0.8), (int)(c1.B * 0.8));
(which should darken it; or, for example, * 1.25 to brighten it)
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