You must be using the dark theme with dark messages.
And therefore, the app forces all text to "off white" (light gray) because that's the only way to make sure there is no "dark" or "black" text which would be unreadable on the black background.
To my knowledge, the Gmail app does not have a dark theme.
And then if you switch AquaMail to either of the two light themes, or even the "dark with white messages", you will see those colors again.
