Find the Odd Disk

I was wondering if it got harder or if it's just random:

    function generateColors(difficulty, blacklist) {
        ...
        let sample = Math.floor(Math.min(Math.max(0, 1-Math.pow(1-difficulty, 1.5)), .99)*5);
        let distance = (5 - sample)/5;
        ...
    }
    function setupRound(blacklist) {
        ...
        const data = generateColors(currentRound/totalRounds, blacklist);
        ...
    }

Plotting that first magic: https://lucb1e.com/randomprojects/js/testformula.htm#%24%28%...

    round#  difference
     0-- 2  5
     3-- 5  4
     6-- 9  3
    10--13  2
    14--20  1

The "blacklist" parameter prevents that you get the same challenge twice. Note also that it submits every answer to the server (fine imo, but I think it would be even nicer if this was mentioned on the page)