That is fantastic, thanks for putting the effort in to make it!
Unfortunately, there's no release schedule for the colour volumes. Usually we find out about a new one less than two weeks before it releases when Amazon.jp or somewhere else finally puts it up for preorder. The rate is inconsistent too. Sometimes they come as often as the black and white ones, other times it can be seven or eight months between releases (like now). But hey, sometimes we get two or even three volumes at once.
Please, it's the least I could do. Thank you for the huge amount of effort curating this collection and then uploading it online for us.
I see. No worries, then. It makes sense that they wouldn't release digital color versions when the black and white ones are still selling well.