iOS 11 Control Center is a noteworthy changes and customizable Apple has ever made. but That is you cannot customize the Control Center color and has no color option by default.
But jailbreaking your iPhone and installing the Cydia tweak called ColorMyCcModules lets you to customize and colorize iOS 11 Control Center as you like.
Developer iKilledAppl3 has made the ColorMyCCModules tweak in a beautiful way which tremendously looks good changing the color of Control Center. In fact ColorMyCCModules is available for free in Cydia.
After installing ColorMyCCModules, you have an option to enable or disable the tweak and have ability to change the various color from the color picker you like.
So recommend you to install the ColorMyCCModules and customize the color of Control Center in iOS 11.
To get this tweak, make sure you have jailbroked your device. If your device is not yet jailbroken follow this guide to jailbreak your iPhone running iOS 11.