Format Property - Microsoft Access

In Microsoft Access, custom number formats can have one to four sections with semicolons (;) as the list separator. Each section contains the format specification for a different type of number. If you use multiple sections but don't specify a format for each section, entries for which there is no format will either display nothing or default to the formatting of the first section.

Section	Description
First	Positive Numbers
Second	Negative Numbers
Third	Zero
Fourth	Nulls

Example

$#,##0.00[Green];($#,##0.00)[Red];"Zero";"Null"

As an alternative, consider using Conditional Formatting. In Access 2007, Conditional Formatting is available on the Design tab; in Access 2010, it's on the Formatting tab.

Conditional Formatting button in Access 2007