Based on this answer do tzitzit have to be white?

Where is the source that they should be?

I know that they are supposed to have at least one blue (techelet) string, but even that is disputed nowadays.


Question: How much choice should I give users?
Answer: 
  How much choice should I give users?


As little as possible that can net as large users as possible. As a rule of thumb, don't add choices when it does not matter. 

If the selling point of your app is productivity, only add choices when it affects workflow significantly. Again, don't add trivial customizations, things like position of buttons, colors, etc usually does not really matter.

If the selling point of your app is customization and personalization (most apps should not have this as the main selling point), things are a little different, put a set of different options as complete "themes" instead of individual options. Most people cannot make good themes, even if they think they do; you'll be having people walk around showing off your app with their crappy customizations. 

If the selling point of your app is ease of use, then less is more. Less means everyone will be using the same app the same way and there is less mine traps for the less proficient users.

Giving too many options is often born out of laziness, you're not doing your user research enough to actually know what they really needed so you give everything.