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 â†’ 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 â†’ Flatten Transparency...) or rasterize (Object â†’ 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 artboardâ€”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 RGBâ€”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)