LEDs Randomly smooth transitioning, accounting for black or how-to switch using HSV
Black will basically be when all the colours have reduced to 0. That can only happen when all three of C1, C2 and C3 are 1. So you basically need to ensure that never happens.

And that is simple enough to arrange, with a "try again" construct:

if ((((Col1 == 0) || (Col1 == 250)) &amp;&amp; (((Col2 == 0) || (Col2 == 250)) &amp;&amp; ((Col3 == 0) || (Col3 == 250))))) {

    // select a random number from 1 to 2 for the C1,C2,C3 array
    do {
        C1 = random(1, 3);
        C2 = random(1, 3);
        C3 = random(1, 3);
    } while ((C1 == 1) &amp;&amp; (C2 == 1) &amp;&amp; (C3 == 1));
}


Pick three numbers. If they are all 1 then you are destined towards black. So go back and pick three new random numbers. Keep trying until you get at least one result that isn't 1.