Using a graphics program, it doesn't look like there is any actual difference in lightness between the summer and winter frames. The RGB values for Hoshino (231, 231, 231) and Suzuki (218, 218, 218) are the same in all the frames. :-)
Let's check it out then!
This is Summer!
W-
Well
~?
You see what I mean?
Yeah~
I do...
It's a feast for the eyes~
Tremble
Hey
~!!
Winter Dark Skin
Not sure what to conclude from this !
We
should be
lighter
than in
Summer,
right?
*Dead of winter.
 Flash
Flash
Shiver
No way,
it's not like that
Hoshino and Suzuki are dark-skinned even in Winter huh?