The current STSI got me thinking

[Ahvae'el]

The soulstone in Diablo 1 was red. In the D2 cinematics it's yellow, but in Tal Rasha's chamber I can't really remember what it was. Perhaps it turns yellow when it's corrupted, like Baal used it. However, I do recall that in D1 the soulstone was red, and that was Diablo's. The soulstone for Mephisto is blue, and thus, it is very very very likely that the next primary color, yellow, is Baal's. Like the other guy said, I think it was probably just bloody in the picture. Otherwise it was a minor oversight on Blizzard's part.