With the !ifset command you can test if a symbol was set in the preamble or via command line. The !ifnset tests if a symbol is not set.
As you know you can set symbols via command line by using the option -D and by the command !set.
The following example shows how to test the symbol `UseColour' and to display the right image:
!ifset [UseColor] !image colour/tiger !else !image mono/tiger !endif
If you call UDO with passing the option -D UseColor or if you set the symbol with !set UseColor inside the preamble UDO will display the color image instead of the monochrome image.