it looks like you are assuming, that App Inventor can only create 15 colors using the colors block?
no: you can create all colors, see the screenshot below
as example let’s create color aquamarine
and you also can use just an integer value, in the example the value -8388652 or 4286578644 to get color aquamarine, see again the link I provided earlier App Inventor Color Chart
see also Juan Antonio’s post here about colors https://groups.google.com/d/msg/mitappinventortest/i_5h0Qua-Q0/HYVpsPFYBAAJ, his study is in spanish, so what about taking a look there?
Taifun