You will need a formula for that. Replace your existing «tests_wcorrect_results_for_merge» field with field coded as:
{=«tests_wcorrect_results_for_merge»-«tests_performed_and_evaluable_for_merge» \# "'«tests_wcorrect_results_for_merge»';'«tests_wcorrect_results_for_merge»'"}
where the second instance of «tests_wcorrect_results_for_merge» is coloured however you'd like it to appear when less than the «tests_performed_and_evaluable_for_merge» value.

Note: The field brace pairs (i.e. '{ }') for the above example are all created in the document itself, via Ctrl-F9 (Cmd-F9 on a Mac); you can't simply type them or copy & paste them from this message. Nor is it practicable to add them via any of the standard Word dialogues. Likewise, you can't type or copy & paste the chevrons (i.e. '« »') - they're part of the actual mergefields, which you can insert from the mailmerge toolbar. The spaces, single quotes and double quotes represented in the field construction are all required.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]