Unless the colour is to be varied you can set the text colour of a control using
%tc before the control when the window is created, possibly followed by another
%tc to reset it after the control has been specified.
A call to SET_CONTROL_TEXT_COLOUR@ can be made in the callback function for the
control in question. This means, for example, that it is possible to construct
a control that displays text in red for negative values and black for positive