I agree that checkboxes are a much friendlier way because as you say, you need two clicks to enter a multiple choice box, one to open the choices and another to select.
Grid Edit does not support colour, but you an have a color message calculated by a formula Rich Text field and I believe that it will dynamically recalculate in Grid Edit mode in your use case. (because the formula only depends on data on that record and not lookup or summary fields.
You can then enable Custom Data Rules for the table in advanced settings and display that error message and block the Grid Edit save if too many choices were selected.
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Mark Shnier (YQC)
Quick Base Solution Provider
Your Quick Base Coach
http://QuickBaseCoach.commark.shnier@gmail.com
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