In mash up and texture packs the sky still remains vanilla. Specifically the Minecraft Halloween mash up and Minecraft Halo Mash up packs.

So, for the Minecraft Halloween mash up the sky is supposed to be purple with clouds shaped like spooky things jack o' lanterns etc. with a big jack o' lantern sun if I remember correctly. Minecraft Halo mash up the halo or ring in the sky isn't there etc. The night skies also show up as vanilla Minecraft skies. These aren't the only mash up and texture packs this is happening to either.