You'll want to create a summary field which is # of Shift Sign ups. This will simply count the number of child records.
You can have a formula that simply says when [# of shift sign ups] = 2, label it full. That formula can be a formula txt and say 'Full' or a formula checkbox.
As far as reports with colors, you can use the color coding options on the bottom of the report to simply say, if([# of shift signups]=2, "hex color code")
If you get the summary field, and have trouble with either step, you can reply back with exactly what you are looking for (I want a field that says 'Full', etc).
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Mike Tamoush
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