I still stamp physically and scan electronically. I'd be interested in hearing what is acceptable as a digital stamp.
Not much is being scanned or printed in black and white these days. All of my stamp inks are black. So with that in mind that my scanner is color and my pdf is color, I always sign with a color. To me this is only one method of ensuring that the copy being viewed is original and not manipulated. If I see the signature as black, it would clearly indicate a copy.
I typically choose to sign in blue. Red ink is for markups. Red can also have cultural connotations where it is seen as a representation of blood.
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Chad Morrison P.E., M.ASCE
Professional Engineer
Greenville RI
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