How to customize the APK file name for product flavors?

Try to put this in your android closure of build.gradle

buildTypes {
    debug {
        // debug buildType specific stuff
    }
    release {
        // release buildType specific stuff
    }
    applicationVariants.all { variant ->
        if (variant.buildType.name.equals("release") &&
            variant.productFlavors[0].name.equals("green") &&
            variant.zipAlign) {
                def apk = variant.outputFile;
                variant.outputFile = new File(apk.parentFile, "green.apk");
        } else if(variant.buildType.name.equals("release") &&
            variant.productFlavors[0].name.equals("blue") &&
            variant.zipAlign) {
                def apk = variant.outputFile;
                variant.outputFile = new File(apk.parentFile, "blue.apk");
        }
    }
}

Now the outputs should be like green.apk and blue.apk.