The origin of the color change might be set by the environment, may be the toner just accentuates it. Or the toner temperature can probably be playing a role - speeding or slowing the toning reaction, resulting in different extent of toning for a given length of time. I am assuming your winter temperatures are lower than summer ones. I have wild extremes in my work space so I have made myself a constant temperature / humidity chamber where I keep my paper for at least a day prior to use. I found it helps in cyanotypes - at least one variable less to worry about.
When I did my POP printing, the color did change (check the link below) with a couple of them much more neutral (which I liked better,) but I could never figure out how to reproduce it. Par for the course for alternative processes, I suppose. I would be happy if at least tonal range does not get affected appreciably from season to season...
:Niranjan.