ColorMeter is simply an app that allows you to plug a probe (including inexpensive X-rite varieties) into your phone and measure many attributes of your lighting. Not only do you get color temperature and exposure information, but you get spectral readouts and color accuracy including CRI and TLCI readings. All this for a $100 app is quite impressive. I use an affordable ColorMunki spectrometer and i1Display Pro 3 colorimeters but there are many devices available. The meter simply plugs into the Android phone via an OTG cable. I keep an old Android phone around just to run Graem Gyll’s Color Meter application.
At NAB 2017 I compared the performance of this combination to a much pricier Sekonic and was blown away by the performance of this application. It is less convenient to launch an application and plug a meter in that to operate a simple handheld device, but the configuration options of the app are astounding and the cost is significantly less.