As a quick fix I'm using the activity name as a prompt. This text works as intended.
The greyed out text is coming from Base theme for all themes.
This looks to me that it is as you have found Andrew, that it is in the way you set the conditions. Which leads me to suspect that there are options which if set wrongly become mutually exclusive. So there will always be potential for errors to creep in.
I am experiencing the same problem with conditional activities. Site has upgraded recently to 2.5 and this has only started happening. I have used conditional activities successfully in the past.
I have just checked courses that have been built before the upgrade. They were working fine before the upgrade, now the conditional activities are not working as expected.
Mary, can't quite follow you. Is there a set of conditions that works and one that doesn't?
Possibly, I'm not really sure it was only a hunch. If two things are set to be opposites of each other at the same time, then that cannot work as one excludes the other, so the item remains greyed out, because the conditions have not been set, although they have, if you get my mad logic?
I will try and get someone to look at where all the logic is added in core and see if they can find out what's happening. It could be a CSS selector restyling it.