How to change tiles' background color in Windows 8.1 start screen?

Yes I was actually referring to the tiles background which was more uniform with the background in 8 and now is not in 8.1

No issue with the rainbow wallpaper :)

Can't help you with that unfortunately. They're all dynamic and are supposed to contrast with whatever background you're using.

You could try experimenting with different desktop backgrounds and set that as a background to the tiles although having said that, I don't see any difference on my own machine even by using photos I took on vacation as backgrounds.

Nah the problem is the thing I'm talking about was already there and they removed it.

Remember when you first booted Windows 8? You could choose among a list of predefined color schemes. Most of them had a let's say light blue square with a smaller, lighter blue square inside it(or orange, green, yellow).

Then you had the ones where all the larger squares were grey and the inside ones were colored.

Now it's like having this second option only, the first one actually made everything look more armonic in Windows 8 so I wonder why they removed it.

I bought my laptop on 5 Feb this year and upgraded to 8.1 the next day so I didn't experiment at all with the tiles.

You could try experimenting as described in the following article. You can see the tile background color changes from orange to blue. http://www.askvg.com/how-to-change-icons-and-text-labels-names-of-start-screen-tiles-in-windows-8/

There are many icons you can use at this location too: %SystemRoot%\system32\SHELL32.dll

There's an article I came across a while back which talks about using the images in the imageres.dll file which is located in C:\Windows\System32. Maybe have a look at that: http://www.makeuseof.com/tag/how-to-find-and-use-the-wallpaper-and-lock-screen-images-in-windows-8/

I saw that and I know changing background helps. The problem is I should be free to set the wallpaper and backgrounds I want and have the tiles behave like they always did.

I hope they'll patch this, I'll just wait for a while and then simply revert back to 8 or 7.

Maybe I'm too concerned about this issues but well, they do look horrible and the fact I work with images, colors, photography and graphic doesn't help me live with this rainbow UI at all.