Grist doesn’t support conditional formatting yet. But it’s definitely looking like a common request, so might be something for us to add to the roadmap.
Meanwhile, besides this idea (Colour Code Table Cells - #5 by dmitry-grist), one other idea is to replace values with a colored emoji, which can be done using a trigger formula. Here is an example: https://public.getgrist.com/vmoDyMSM7Ri6/Red-if-Empty/m/fork
If you delete a value in a cell, it will get a red square in it instead of staying blank.