>this.column4.DynamicBackColor = 'iif(duedate < DATE(),RGB(255,50,50),this.column4.DynamicBackColor)'

Try this instead:
this.column4.DynamicBackColor = 'iif(duedate < DATE(),RGB(255,50,50),'+this.column4.DynamicBackColor+')'

This is assuming, that this.column4.DynamicBackcolor is already set to some expression resulting in a color.

If it get's too complex, maybe create a function in the form, that you call from DynamicBackcolor:
this.column4.DynamicBackColor = 'thisform.columncolor(duedate)'

* thisform.columncolor method:
LPARAMETERS tdDueDate
...
Return RGB(...)

Bye, Olaf.