Consider creating a calculated field (called "Highlight") on the measure that is providing the rank. You may find that a Sum (as used in my example formula) is not appropriate - perhaps a Min or Avg is better for your situation. The calculated field should look similar to the formula below:
IIf(Sum([Rank Field])<=5,"Rank","Not Ranked")
Once "Highlight" has been created, move it to the Color shelf. Modify the colors such that the "Rank" values have a color you wish for them to have, and set the "Not Ranked" color to black (or the same color as the text in the rest of the table.
Thank you Andy! The formula worked just as you said. Tricia