Designing forms for accessibility involves considering some additional guidelines for using color. Colors can emphasize and enhance certain parts of your form, but you should not convey information by color alone.
For example, a red asterisk often indicates that a text field requires user input. Because users with vision impairment may have difficulty seeing the color red, a better choice for accessible forms is to set the text field type to User Entered – Required and define a message that indicates that the field requires input.
Many users with vision impairment rely on high contrast between text and the background to read the form. We strongly recommend that you use the default font and background colors, Black on a white background provides a high-contrast that improves the readability of the form. If you must change these default colors, ensure you choose an appropriate combination of high-contrast colors.
Design your form to interact normally with other applications and system standards, including support for standard Windows Control Panel settings for colors. Users with vision impairment or those who are color blind may have difficulty seeing the cursor. Use standard settings for color may help eliminate this issue.
While designing your form, test it frequently using a color scheme setting similar to what many users with vision impairment will be using to complete your form. This practice helps you discover and correct issues early in the design process.
See also 

Color in accessible forms