You can't change the color of a stamp via the Properties Bar. You'll need to create a new custom stamp with the color you want to make that change.
If the stamp doesn't print make sure you've selected "Document and Markups" or "Document and Stamps" in the "Comments & Forms" option of the Print dialog:
Well, that worked. Thank you. However, I have a comment about Acrobat. It is the same comment I have had with Word. I have been using Acrobat and Word since the mid 1990's. Every time I upgrade I go through this learning curve that drives me crazy. I have been doing things like changing colors on stamps, editing pdfs, etc. but with each new upgrade the tools are in a different place. I spend endless time going on forums trying to do things I did in my sleep a year ago on a previous version of both Acrobat and Word. I recently attended a workshop on Acrobat Pro XI and the instructor told me that when Acrobat DC comes out (which it did and I am now using) he would have to spend 60 hours reworking his all-day workshop.
Does anyone share my feelings or can anyone explain why they keep changing things?
My theory: If you bought a version that looked and behaved exactly like the previous one you'll feel like they cheated you.
So they have to move things around and change the way they look so you feel like you get your money's worth for the spanking-new version you bought.