Grey out first two segments in charge account on PO distributions

Hi,

Our client has a requirement which is as follows.

The Account code combination(COA) being used is a 8 segment value. This code combination will be used on requisition and purchase order distributions.

They want to grey out first two segments in the charge account field on purchase distributions. However, they want other segments enabled so that those values can be changed by Buyer depending on requirement.

I was able to make the whole charge account non-editable with form personalization. How can we achieve such kind of partial grey out of field?

Thanks,

VB