I ran into the same issue. I even re uploaded the exact same image the was uploaded a month ago and the colors are now different. The issue occurred for me on blues similar to yours Mxm.
When comparing the original file to the version after the squarespace uploader handles it, I'm clearly seeing that the SquareSpace uploader is adding saturation to the image.
I found that uploading the image, and then using the SquareSpace image editor to turn down the saturdation by 10 got me very close to the original.
I also found that using a jpeg instead of the png fixes the issue as well. It seems like my PNG files are getting compressed upon upload to squarespace.