It depends on what you want Avraham and what is possible.
It is impossible to change the background color or individual items in a ListPicker with Blocks.
You can add a colored emoji image to what is posted in the ListPicker.Elements without modifying your original List. See my previous posts for a method to add emoji to a ListPicker (item) as a flag or use an extension with a ListView instead or write your own ListPicker extension.
Early in this discussion it was pointed out that it is impossible to change the background color of items in a list picker using Blocks. Several people have shown ways to work around that using different controls and gimicks.
What is, is.