There is no proper hint and difference between header and interactive buttons in all four screens. Giving grey color for header and leaving same accent color for button will help users to find where to take actions without much difficulty.

On first screen :No seperation in between Login button and fields and no proximity followed between text and field. Low contrast for login text in the button.

On second screen: Should change order, start from email, username, password and confirm password. 

On third screen: So much gap between button and tabs at the bottom that stops eyes to look down. Can make welcome test bigger will help fillup the space and easy to read.

On fourth screen: No fields issued to input the texts. It's not necessary for the text for back icon.