The image for the dialer and phone buttons are in com.htc.resources.apk /res/drawable-hpdi folder
called htc_pin_asset_pressed.png and htc_pin_asset_rest.png
To change the button text color it is in the phone and dialer apk's. If you decompile the apk's you can make the following edits and recompile it with apktools. No need to sign them.
HtcDialer.apk
Unless you use an existing color a new color needs to be added to:
/values/colors.xml
and used in:
/values/styles.xml
Example=<item name="android:textColor">@color_______
Phone.apk
It is only in /values-hdpi/styles.xml
I hope this helps.