An important aspect of color on the Web for both low vision and color blind users is sufficient contrast between the foreground (text or graphics) and the background. Maximum contrast is black versus white, but this combination can be considered too overwhelming.
Many modern designs are so "subtle" that the contrast can actually be insufficient for some readers. If you plan to build your lessons using a subtle color palette, it is recommended that you use a color analyzer to ensure that the contrast is sufficient.
In most online tests, you enter in the hexadecimal color code for the foreground (text) and background colors, and the tester generates a numeric result. Usually a low number or ratio indicates too little contrast.
mAuthor lets you define the look for every addon in your lesson making it possible to define the proper contrasts and allows adding other means of conveying the information about the correctness of the activity. For example, you can add the check mark and X mark next to the answer. This lets you meet the Success Criterion 1.4.3 – Contrast (Minimum) and Success Criterion 1.4.1 – Use of Colour.
You can find more information about the CSS classes in the documentation. To find the names of the CSS class for each module, you can check the documentation of that module.