Overlay colour changing based on background colour

Hi there, so I've come across an issue with the colour overlay feature in photoshop and I'm hoping someone can help. I have a pattern that is on a white background, I am using a colour overlay on the pattern to blanket change the colour to blue (C:100 M:82 Y: 25 K:10). Now even though the colour overlay is set to normal and the colour is showing as the above blue values, it doesn't appear on the screen as that colour. When I use the pipette tool on the pattern it shows that the colour is actually a blue-purple: C:67 M:54 Y:17 K:7, even though the colour overlay function is showing the colour as Blue: C:100 M:82 Y: 25 K:10.

To make things even weirder, when I fill in the background with the blue, the pattern overlay then changes its values and matches the blue background and is no longer the purple values when I check via the pipette too. I have also tested this with varying different shades and each time the overlayed pattern will change its values depending on what background colour has been used. 

Hopefully, someone can help with this as I can't get the overlayed pattern to be the colour I need it to be and it's becoming frustrating. 

Thanks