How does FG determine what to put where?
Actually, you do - it's shader generated snow, and since shaders aren't co-ordinated, the transition shader has a different snow effect from the landmass shader, they make different assumptions about lighting, they do not agree with permanent snow,... So every landclass which uses transition effects gets one type of snow, every landclass which doesn't gets another kind of snow.
Try --materials-file=materials-dds.xml, this should use a more consistent shader set. Or switch off transition effects and keep only landmass shader. Or disable all shader-generated snow and use --season=winter at startup - looks much nicer in my opinion anyway if all is snow-covered.
The issue itself seems to be that
1) sun is very low, thus
2) the light is yellow to organge, and since
3) one shader assumes that snow inherits the light color, the snow gets yellow (or probably red at sunset)
4) another snow shader does not make that assumption but keeps it in grey shades instead, you get the color mismatch
If you feel lucky, you can go edit the shaders