Why do you want to grey it out? Do you want to make it inactive, or just color it?
If you want to make it inactive, you click on the field in design view, go to its properties dialog box, then click the Data tab, and select Enabled = No. This will grey it out.
If you want to make this conditional based on some value (in a record, for example) then you check the condition, and use:
Me.TextBox.Enabled = False
I am pretty sure that last line will work. The code may be slightly different.
Does this help?