tmpRange.Find.Replacement.Font.Color = Color.FromArgb(100, 150, 75);

Color c = Colors.Blue;
var wdc = (Microsoft.Office.Interop.Word.WdColor)(c.R + 0x100 * c.G + 0x10000 * c.B);

using Microsoft.VisualBasic;

int rgbColor = Information.RGB(100, 150, 75);
Word.WdColor wdColor = (Word.WdColor)rgbColor;

public static void ColorRGB(this Wd.Font font, int red, int green, int blue)
    {
        font.Color = (Wd.WdColor)(red + 0x100 * green + 0x10000 * blue);
    }