Read the MSAccess Help:
Change the color of the control that has focus on a form
Open the form in Design view or Form view.
Make sure that the control is selected, and then, on the Format menu, click Conditional Formatting.
In the Condition x area of the Conditional Formatting dialog box, click Field Has Focus in the first box.
Select the font style, color, or other formatting that you want the field to have when it has focus.
Note When you open the Conditional Formatting dialog box, it displays the current default formatting for the selected control, including colors, font styles, and any expressions that you may have defined for the field.
OR
Write the VBA Code in the On Current Event to set the border type property of the fields to solid or change the color property.