Hi Mollie,
Yes, unlike regularly grouped reports, matrix grouped reports never have allowed such textmode augmentation. I have a vague memory of kickstarting such a report out to Excel, fiddling with it (effectively in text mode), then kickstarting it back into Workfront as a hack (and wish you luck if you Go There), but instead, suggest you consider adding a second custom calculated parameter (e.g. Matrix Color) with a formula that compares the hours per project against your existing Budget Hours custom parameter, and depending on the difference, then resolves to a text field (e.g. Red if > 110%, Yellow if <=110% and >90%, Green otherwise). With "the math" done in advance in this manner in a single parameter, you can then use the Matrix Report user interface to define conditional coloring using the resulting Matrix Color values (e.g. if Red, show red background, etc)
Regards,
Doug