What widgets are you using for the cells?
Make sure they have a background, and aren't transparent.
Widgets like Fl_Input have a background, but widgets like Fl_Group, Fl_Box, and Fl_Widget by default use FL_NO_BOX,
which means setting the color() has no effect since the box is transparent. If yours is such a case, be sure to set the
widget's box() type to one of the FL_XXX_BOX values. Note that the FL_XXX_FRAME box types are also see-through;
the only thing they draw is a frame.
The reason for the FL_NO_BOX default is to ensure the default behavior of FLTK is to decrease unnecessary drawing,
as often Fl_Group doesn't need to draw a background (to let the window's background show through), and in the case
of Fl_Box, it's most often used for showing labels, and also don't need their own background.