The tax box will go red for 2 reasons:
1. When you create a supplier there is a box you can tick to say that the supplier is registered for GST, but if you do NOT tick this box, and then you have an invoice from them which you're being charged GST on, our system will alert you to the inconsistency by highlighting the tax amount box in red.
2. When you have ticked the box to say that the supplier is registered for GST, our system will also check with the Australian Business Registrar website to see if the ABN that we have in the supplier details is or isn't registered for GST, then when invitbox detects the supplier has charged you GST when in fact they are not registered to do so, the tax box will be shown as red.
Note that if you hover your mouse over the red box, it will display 'Supplier Not Registered For GST'.