White background causes colour shift

Question

Ran into a bit of a strange problem... am sure there's a simple explanation...
Gradient or solid colour in the blue range set at 40% transparency
1/ When placed on a blank surface (no background) looks ok
2/ When placed over an underlying white background gets a magenta tinge
Questions:
1/ Which is the true colour?
2/ What will i get when printing by offset?
3/ The following seem to be the affected / effects:
a/ CMYK Red (0, 100, 100, 0)
b/ CMYK Yellow (0, 0, 100, 0) - NO effect
c/ CMYK Green (100, 0, 100, 0)
d/ CMYK Cyan (100, 0, 0, 0)
e/ CMYK Blue (100, 100, 0, 0)
f/ CMYK Magenta (0, 100, 0, 0) - looks paler
Attaching a test file...
Edit: Here's the jpg

Answer 1

The question now remains is, is this just an optical illusion (meaning visible only on the screen) or is will it translate into print?
That's a very easy thing for you to test yourself, Shunith. Just print seps to a PostScript laser printer or to Adobe PDF.
If you do the latter, and then open the resulting PDF in AI, you'll find that the result is the same whether the screen display option is on or off. If you build the test file like mine, you'll also find that the C, M, Y, K filled rectangles are split into three paths each, where they overlap the white object below. Their grayscale values, however, will be the same.
You'll further find that the black text adjacent to, but not intersecting with the "translucent" rectangles is present on all four seps, but is white on all but the black sep. On each sep, the other three sets of rectangles are present as well, also filled with white.
In short, Illustrator does all kinds of "untidy" smoke-and-mirrors things when it creates PDFs and prints/exports files with transparency features (and when doing live effects like Compound Shapes). You choose to "trust" these convoluted constructs when you use such features as-is.
I would not be at all surprised if someone on AI's development staff said that the display aberation noted in this thread is by design. As I said, I neither pretend to understand, nor would necessarily agree with, the "logic" for it.
JET