When I test the pH of red wine, it stains the test strip. How can I avoid this?

It is not a good idea to dilute the sample with water. While this helps to avoid the color stain, dilution of the hydrogen ion concentration will change the pH.

A better suggestion for measuring pH might be to use activated carbon (like that used in aquarium filters) to help take the color out of solution sample. This method doesn’t appear to affect the pH of the solution.