Define state colors from the start
Every app needs colors for error, success, warning and info. Choose shades that fit your palette and don't use the error color for anything else, like brand buttons. Pair color with an icon or text so it's clear without relying on color alone.
Design dark mode too
Most phones support dark mode, and many users have it on. Inverting colors isn't enough: use very dark grays instead of pure black and slightly lower the saturation of your accents so they don't vibrate against the background.
Check contrast and tap targets
Text should have at least 4.5:1 contrast with its background, per WCAG. Buttons should look different when active, pressed or disabled. Test the app outdoors and with low brightness, since that's how many people use it.