So far, WP7 does not allow users to specify their own custom colors when it comes to Windows Phone 7 (WP7) interface and tiles( themes) . but thanks to XDA developer community there is a way to do this, in case you hated the stock colors. This feature is intended to be part of next major update from Microsoft but till then you can give it a spin on your developer unlocked device.
Neowin has written up a handy guide on how to change the accent colors on your own Windows Phone 7 device, based on the process figured out by member “xboxmod” of the Xda-Developers forum. It involves access to the registry of any HTC device running Windows Phone 7, write their own registry values over the top of some preconfigured ones, such as accent colors.
Read the full guide here