Looks like I may have spoke too soon on my last reply. I seem to have figured it out by using a Computed Field Formula. After some testing it is working pretty well, and thought I'd share the result.
STATUS field - Picklist 3 options (Complete, In Progress, No Action Required)
FLAG field - Computed Field Formula
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CASE WHEN (STATUS = 1) OR (STATUS = 3) THEN '<span style="text-align:center; line-height:30px; color:gray; background:gray; width:100%; float:left; height:20px;">GRAY</span>'
WHEN (STATUS = 2) AND (DATE_DUE > getdate()) AND (DATE_DUE < (getdate()+2)) THEN '<span style="background: orange; color: orange; width:100%; float:left; height:20px;">ORANGE</span>'
WHEN (STATUS = 2) AND (DATE_DUE < getdate()) THEN '<span style="background:red; color:red; width:100%; float:left; height:20px;">RED</span>'
ELSE '<span style="text-align:center; line-height:30px; color:green; background:green; width:100%; float:left; height:20px;">GREEN</span>'
END
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The hardest part is working out the syntax of the code which PLM will accept. Is there a reference in the Help to this? I've looked but have not been able to find anything.