eternal guard are winterbound, and wild riders are summerbound, but it dosent mean that they cease to exist in the opposite seasons
eternal guard still guard theborders of the forests in summer, but in winter that duty falls solely on them, thats wy they are more winterbound. a winter themed army sshould have more eternal guard, while a summer army should have more wild riders.
wild riders in winter guard the kings glade, but still can be used in winter, quercia had an ideaabout making forest spirits slightly stronger and more expensive in spring themed armies:
http://kul.woezies.nl/Asrai/viewtopic.p ... highlight=
maybe you could take a leaf out of his book and make either WIld riders or eternal guard weaker with a cheaper points cost (eg wild riders -1 initiative for -2 points) but still include them in the army if you want everything to figure up fluffiy
or else go on and make an autumn army, but in that case paint wild riders in summer colors so they stand out(i dont think theyd bother changing camo scheme just because winter is coming)