Prepopulated Category Filter Search and Category Coloring in Simplelist?

Is there documentation on how to create and add a filter. I am looking to add one like Demo --> Search

Hello,
Sorry, I cannot answer seriously, if I do not know on which feature you want to apply filters.
Depending on what you want to apply filters, there are many differences.

So essentially I would like to display just category, as a searchable filter. And have it prepopulated with the categories

I guess, you want to apply it e.g. on "Simplelist" and if you select the filter "Category" you want the search field prepopulated in a dropdown list (as alternative to a free text search).
Nice idea!
Currently this is not possible, sorry.

HAha, Thank you for check....

Do you know if it is possible to change the text or background color based on category on "Simplelist"

Currently this category coloring of the background - as usable in the "Calendar" - is not applicable to "Simplelist" out of the box.
One could enhance the the PHP code for that - or - use a custom CSS (even that is not that simple, because you would hard code the coloring - then even for the text - for each category and it would fall to the default for new categories).