Guideline 1.4 Distinguishable: states that it should be made easier for users to see and hear content including separating foreground from background.
About the use of color, it should not be used as the only visual means of conveying information, indicating an action, prompting a response, or distinguishing a visual element.
For example, the required fields of a form should be identified using a particular red icon (i.e.: "*"), and an alternate text saying "Required field." At the top of the form, there should be text explaining the criterion used for the required fields: "Required fields are signaled using a red * icon with an alternate text required." The required fields can be implemented in GeneXus using Text Before and Text After content class property.
The links have to be distinguishable. See How to configure link colors and hovering effects for links in GeneXus.
The text should be able to be zoomed to 200% without losing the functionality of the page. One possible solution is to give the user the possibility of using different versions of cascading style sheets. The Theme can be assigned at runtime using the SetTheme function, so a different CSS will be assigned to the page. This can be done for the Master Page itself.
Another possibility is to use Viewport Units. See HowTo: Resizing font size depending on the viewport size.