I have a simple single-color graphic, on a white background, which I want to use as a background on an Excel spreadsheet. It works fine except that the color is too dark, so it's too distracting.

I tried lightening it but that just made it a lighter shade color, so it was still just as distracting. How do I make it fainter, like 20% gray?

If possible, I'd like to do it with freeware.