Do you mean the app tile itself or the titlebar at the top?
If it's the app itself, I know of the Autodesk Sketchbook app that turned their tile grey for the windows 8 version after it posted a new Windows 10 version. They both work, but I guess they just wanted to make it clear which one they are supporting. If it's for another app, try looking in the store for an updated equivalent.
If it's the titlebar, there are new personalization settings that separates the colour from the titlebars if you want for a more neutral colour (white or grey). You can change this simply by going to the colours option under personalization in the settings app.
Hope this helps!