Is it possible when using a checkbox set to fill the boxes with a color instead of an X
Not with a standard check box format selected for a field in the inspector.
But if you look at some of the "check box" and "radio button" examples in the following file, you'll see some examples where the checkboxes are really buttons and either the Hide Object When (FileMaker 13 and newer) or other methods such as a global field (older versions) are used to produce the same type of check box functionality, but now you have a great deal more flexibility as to the actual appearance of the "check box" and such a box with a fill color becomes possible.
"Adventures in FileMaking #2 - Enhanced Value Selection"
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