One way:

Format field 3 as red.

With field 3 selected, choose Format/Conditional formatting and use
dropdowns and textboxes to read

CF1: Formula is =AND(<field1=0,<field2=0)
Format1: <patterns/green

where <field1 and <field2 are your cell references (e.g, A1, B1)
In article ,
smoran wrote:

Hi,

I have three fields in total ............. I need to make field 3 go
green IF field 1 AND 2 = value 0. I need to make field 3 go red IF
field 1 and 2 = anything but 0.

How do I do this ?

Regards

Ste