Yes. Your app is automatically up to date.
The only things you can’t modify on your apps without new submissions are : app icon, app startup image (splash screen), app name, app description, categories and keywords, and the mobile website url (in settings>domain).
Another point concerns the “audio” feature, if you have bought it: if you want it to work with multitasking (navigate through your phone, outside your app, with music keep on playing), you have to integrate it on your first publishing, if your app is already published when you add this feature, it will not work in multitasking mode, and you will have to submit again if you want the audio to work in multitasking.
But design, colors and of course features are editable when you want and without new publication process.