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
}
}
}






