The messiness is caused by the turtle starting from the direction that the spiral ended in. For the second image we reset the direction, so it looks more organised.
reset
canvassize 500,500 for $y = 5 to 20 { go 250,250 direction 0 if $y ==5 { pencolor 0, 0, 0 } else if $y < 11 { pencolor 105 + ((10 - $y)*30), 0, 0 } else if $y < 16 { pencolor 0, 105 + ((15 - $y)*30), 0 } else { pencolor 0, 0, 105 + ((20 - $y)*30) } for $x = 2 to 20 { repeat $x { forward $y turnright 105 / $x } } }