Check out this video demo to see 
Victor Tsaran, who leads the Accessibility program for Material Design, 
using VoiceOver with the Flutter Gallery web app.

Flutter’s standard widgets generate an accessibility tree automatically. 
However, if your app needs something different, it can be customized 
using the Semantics widget.

Sufficient contrast

Sufficient color contrast makes text and images easier to read.
Along with benefitting users with various visual impairments,
sufficient color contrast helps all users when viewing an interface
on devices in extreme lighting conditions,
such as when exposed to direct sunlight or on a display with low
brightness.

The W3C recommends:

At least 4.5:1 for small text (below 18 point regular or 14 point bold)

At least 3.0:1 for large text (18 point and above regular or 14 point and
above bold)

Building with accessibility in mind