Types of accessibility features
There are many different categories of accessibility features you may need to consider while making your app accessible. You'll need to consider whether your app really needs to account for these categories, of course. But it's important to be aware of the various needs your users may have. Let's go over the various forms.
You'll want to check out http://webaim.org/articles/ that has a lot of information on the various kinds of disabilities and how best to help your users. We've summarized some of the information available from this fantastic resource in the following sections, but you really should review this site in its entirety.
Color vision deficient
Many of your users may be color vision deficient (otherwise known as color blind). See https://en.m.wikipedia.org/wiki/Color_blindness for some statistics on prevalence.
The term color blindness is an unfortunate one; it's not that affected individuals don't see color—many of them do. It's that they may have...