In ID 5.5, I have a character tag for "red" and one for "italic". In the various definitions of both tags all other fields are blank and in the General > Style Settings, Italic says [None]+Italic, and the red tag says [None]+color: CorpRed. So if I apply the italic tag to a word and then apply red, the word turns roman and red ... in other words, it acts as though red is defined to be plain roman. And of course, if I apply red then italic, the red text turns black, even though the italic tag has no color specified.
Is there any any way to get these tags to not trump each other?
Since InDesign uses the word "tag" for markup related to XML automation, I'm going to assume you mean "character style."
The answer is No, you need to take the cartesian product of all possible effects and make a style for each one. In your case, that means you need a "red and italic" character style. This situation is awkward.
Depending on how you need to get there, automation may be able to help.
Depending on if there is an identifiable pattern, you might also be able to use a character style and a GREP style on the same bit of text.
Thanks. In my mind that is a poor design.