Try:
(CASE WHEN [DueDateStart] < GETUTCDATE() THEN '<font color="red"><b>' ELSE '<font color="black"><b>' + viewActivity.DueDateStart + '</b></font>' END)
- I haven't tried this, just a first glance attempt.
- better to use GETUTCDATE() instead, since all times are actually stored on the server without timezone information, so it can be translated and viewed properly anywhere in the world based on where you are.
- Your END was in the wrong spot, and you weren't wrapping in brackets.
BUT - note, using a date in an SQL override is undesirable. The date will come in UTC rather than converted to your local time zone, and it will come unformatted. No easy way around this.