When you turn a layer into a 3D layer, it will respond to any lighting in the scene. If you don't want that, simply turn off the Accept Lights option and so on.
i turned off Accepts Lights, Casts Shadow, Accepts Shadows and also set all parameters to 0% (and later to 100%) in Material Options but it did no solve the problem. There is no Light layer in my comp
Is the blood the right color if you disable the stroke effect?
Is the blood layer in the exact same spot in 3d space as any other layer?
Well, if i disable stroke effect, the blood disappears - because it's not a layer on its own, but simply an animated mask path. All the layers in my comp are in the same Z position.
However, the problem solved itself after i opened my project after some 48 hours (i had tried restarting AE and rebooting PC before posting this thread, but it didn't help). I did absolutely nothing, not even touched any layers in the comp and now the real color is back