Robert,
I assume InpField1.rawValue = NULL is pseudo-code.
Try the following in the initialize event of InpField1:
To set the color to grey
InpField1.ui.oneOfChild.border.fill.color.value = "190,190,190";
To set the access to read only
InpField1.access = "readOnly";
To reset access to user entered optional change the access property to "open" on the enter event.
Many examples can be found here
http://www.adobe.com/devnet/livecycle/designer_scripting_samples.html#changing_appearanceSteve