Adobe Illustrator CS6
After setting the document dimension and bleed, I realised that the default white artboard won't go into the bleed region (5mm), and that remains as grey. So, almost out of OCD, I made a pure white shape stretching towards all the bleed borders and set it in it's own locked layer, like a white base, for all of my 200 artworks in a particular project.
I didn't think that this would be an issue since it's white. All the artwork were placed in their own layer above this "base layer".
However, I came across some shapes which were red (0,100,100,0) CMYK. They looked fine until I applied a 75% opacity. So, they look reddish when overprint preview is OFF/ON, and pinkish if overprint preview is OFF without a white base.
Why is this?
Am I missing something funadamental about print. Should I leave the white base layer in my 200+ images or go through all of them deleting the white base? They will all be linked into InDesign for a book design.