You cant really go by colour for the monsooned malabar because they are really light coloured. I stop mine at the start of second crack (which I think is best for this bean) and the colour is around CS4 or 5. I roasted a batch up to rolling second before and it was at CS6 colour. Really light coloured bean. Id hate to think how far I would have had to go to get to CS8!
I use a corretto for my roasting but started out a long while ago on the popper, I find cracks easier to hear on the popper than in my partially covered corretto but that shouldnt be an issue with the malabar because it has really loud cracks, almost firecracker-like... Id suggest after firstcrack, you keep your ears out for the more subtle second crack and pull it straightaway when you hear it.