Arduino function to fade from one RGB color to the next
There is no delay in there. It will fade very quickly, add a delay e.g. 5ms with n = 200 for a 1 second fade:

int n = 200;
for (int i = 0; i &lt; n; i++)
{
    Rnew = Rstart + (Rend - Rstart) * i / n;
    Gnew = Gstart + (Gend - Gstart) * i / n;
    Bnew = Bstart + (Bend - Bstart) * i / n;

    // Set pixel color here.
    strip.setPixelColor(0, strip.Color(Rnew, Gnew, Bnew));

    delay(5);
} 


The other thing is your last value of n in the loop is 199, not 200, and thus Rnew etc don't have the correct final value. Change the comparison to be &lt;= not &lt;:

for (int i = 0; i &lt;= n; i++)