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I have a problem. I'm working with an object at 50% opacity. When it's over the artboard itself the color looks great, but when I put a white object behind it I get this weird color that doesn't look accurate.
This happens with every object and only in CMYK. I'm pretty sure this never happened to me before so maybe I changed something in the settings or I don't know.
Does anyone have any idea?
I'm not 100% sure what's going on here but I can tell you that what you see blended with the white object is actually the correct color. If you do as you did in your example with a 100% K square, you will see that the part over the white object is 50% K while the part blended with the artboard is something else.
If you turn on overprint preview (View → Overprint Preview) you will see no discrepancy and artwork blended with the artboard will look as it does blended with white (i.e. what you though wasn't accurate). If you select your artwork and flatten transparency (Object → Flatten Transparency...) or rasterize (Object → Rasterize...) you will see that it corrects the color to what you see when blended with white.
Now, weirdly if you change your document to RGB you see the color as it was over the artboard—not over the white object. But, if you flatten transparency and switch back to CMYK (so your color looks how you initially wanted) you'll see the values are wrong. So it seems like Illustrator works out the preview for transparency over the artboard (i.e over nothing) in RGB—but as I said before, I'm not really sure.
Also (I just checked), if you go to document setup and turn on "Simulate Colored Paper" and set the color to white, you get the correct colors (the same as you normally get when blended with a white object).
Basically don't rely on transparency blended with your artboard.
(Also, I'm using CS6 so this isn't new in CC)
Never noticed this happening but I just opened my illustrator and replicated what you show on your image and mine does it too. The only logic explanation I can come up with is that the programme's way of "thinking". And this is just a guess but, if you have no objects behind it the colour does not change because the white of the artboard is considered as "nothing" - not an object and no colour.
As oposed to when you put a white square behind it, telling illustrator : this is an object with a white colour.
In CMYK mode it will take vibrance away from your blue because it replicates the behaviour of colours as pigments and not as light. If you change is to RGB you can notice that it also changes a bit but because RGB adds light to the colours the difference is not so evident.
Image of the colour difference when you change the document settings to RGB below.
It happens because the Layer is in blending options like Multiplicate
I think what’s happening here is that when you put a white object under a transparent one, what is being used to calculate the resulting blended color is the paper color that is set by your document’s color profile, and not the “pure white” that your artboard background is appearing in. Because “white”, when printed, can never be lighter than the paper color.
The same effect is visible by either turning on overprint simulation or soft-proofing with the simulate paper color option being set.
So if you assign a color profile to your document that is based on a rather white paper, the effect won’t be as visible as when using a color profile with a darker paper color, like a newspaper.
If your main intention is to use the file digitally, you should set the color space (Adobe Illustrator) or transparency blend space (Adobe InDesign) to RGB. By doing that, when you apply / flatten transparency, the RGB color space is being used and the paper color of the CMYK color profile is ignored.
