When Apple released iOS 7 they included some tweaks to the Mail app including a way to customize message flags to suit your tastes. If you aren't a fan of the default flags that appear, there's now another option. Here's what it looks like and how to change it:
How to change the email flag stay on iPhone and iPad
Tap on the Settings icon on your iPhone or iPad.
Scroll down and tap on Mail, Contacts, Calendars.
Now select Flag Style under the Mail section.
Here you can choose between either the Shape or Color option. Choose the one you'd like.
I've never been a huge fan of the flags and was glad when Apple finally provided the option to choose something different. I much prefer the standard shapes. Let me know in the comments which you prefer!