Maintaining and using cell highlighting
Using Excel 2010 for this example, if you highlight the cell with a color that means "input needed" (lets say red), then the conditional formatting rule could be "ifnumber()", "iftext()" or a formula that confirms that the input is in an expected form with a different color (say green). Now the cell appears red by default (needing input) and green when filled in, continuing to show a highlight indicating where user input is allowed. Since multiple rules are permitted to apply to a single cell, you could even change the highlighting to distinguish between valid input and invalid input or any other condition you can put in a formula like highlighting if the input is outside an expected range.
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