ComboBox.GridlineColor Property (Access)
Gets or sets the color of the gridline for the specified combo box. Read/write Long.
The GridlineColor property setting is a numeric expression that corresponds to the color you want to use for a control's gridline.
You can use the Color Builder to set this property by clicking the Build button to the right of the property box in the property sheet. Using the Color Builder enables you to define custom border colors for controls.