I tried to add html tag in the "Prompt Text", but it does not work.
There is no fully dynamic way to change color, font or other properties of input controls.
There are two supported ways for now:
1. Add css class for input controls definition in jasperserver\WEB-INF\jsp\modules\inputControls\InputControlTemplates.jsp - this will change look&feel of controls in all reports which use default report parameters form. (works for JasperReports Server of version 4.7.1 or later)
2. Change report parameters form for specific report(s) - go to Edit report -> Customization -> JSP Location
and set path to custom parameters form jsp (you can copy&paste DefaultParametersForm.jsp and customize it). This way allows you to customize input controls only for specific report(s)