any ideas how to create quick alpha mask (matte) see the attached image

I was working with this animation and i needed to create alpha mask just for the mobile phone screen which is in blue color, it is only blue color in this scene so i thought i can use some color correction effect to specific just this area of the composition. I quite figured it out with the Change Color effect and Solid Composite bellow it, it creates the matte i attached (all black and white matte i attached).

Is it only way how to achieved this or there are other ways? Please let me know what you know guys! Cheers!