You want a calc field that returns a container, and a series of global container fields. In each global container field, insert a bitmap graphic that is a solid color. The calc field will return the contents of one of the global fields, depending on the entry field (I suggest using "Case").

Put the calc field behind the entry field, and make the entry field transparent.

I'm not quite sure what you mean by the "field changes color" would this be the color of the text in the field or the color of the field background? I'll assume text in the field.

What you need to create is two calculated fields to put on top of your existing field. The top field needs to be formatted with the fill color as transparent. The new fields are:

GreenResult (calculation, text) = Case(EntryField = "x", EntryField, "")

RedResult (calculation, text) = Case(EntryField <> "x", EntryField, "")

The fill of the top (say GreenResult) field is transparent

The fill on the middle field (say RedResult) is white.

Both are set to not allow entry (I know you can't enter into a calculation field) but this forces you to EntryField on the bottom when you click on the stacked fields.

-bd

This link provides for the field background method www.filemaker.com/ti/104445.html

Check out the sample file (Handy Bits) I posted in the samples section of this Forum. It contains examples of both colouring options you are looking for - and also combining them - under its "Colour" Tab.