Less is more
Being DRY means Don’t Repeat Yourself. Following this primary principle can keep you safe in the long term.
Stick with 3 colors
Try to use 3 main colors at max to maintain a perfect balance. Play with shades and tones if you need diversity.
Follow the 60% / 30% / 10% rule for the best color scheme balance. Use 60% as your primary color, 30% as secondary, supporting the main one, and 10% as the color of attention, used for CTA, etc.
Maintain a hierarchy
Avoid using too many fonts, their sizes, and inscriptions. Create a consistent visual hierarchy across the whole app.
2. Different fonts/inscriptions in use
Exception: This advice works for interfaces, commercial and technical projects well. Design magazines and blogs offer sets of headlines/paragraphs with 2-3 various fonts/inscriptions.
Exception: You may use bold to highlight important phrases. Don't overdo it, all right?
Create consistent rhythm
Create a consistent interface rhythm. Document spacings and use them with the same consistency, from page to page.
Multiplication of entities is a common mistake. It is terrible when multiple different components perform the same function. It brings more technical debt, UX and scalability issues.
Know their rules
Google and Apple got totally different visual guidelines. As well as their user’s habbits and experience. It is imperative to consider its philosophy, principles, navigation, appearance, etc.
Apple Human Interface Guidelines
Google Material Design