I've also built a light sensor for my home automation project but I was never really satisfied with it. The light level is different in summer and winter but still you might want to drive your shades. Also during dusk and dawn it was difficult. And you need to make sure that the lens is always clean or it is not working properly...
I have a new solution which works imho much better:
two temp sensors. One in direct sun, the other one in the shade. I look for the delta temperature and based on that can tell if sun is shining or not.
using the water proof dallas sensors for that. easy, cheap, reliable.

SJ

or UV index sensor
https://www.aliexpress.com/item/GY-8511-ML8511-UVB-UV-Rays-Sensor-Breakout-Test-Module-Detector-Analog-Output/32597000768.html

Yes, I use a low cost (10 cent) Light Dependent Resistor (LDR) to measure ambient light levels. The wiring is quite simple, one end of the LDR to ground and the other wire connects to a 10k Ohm resistor. The other end of the 10k resistor goes to +5 or +3 (whichever you are using). The junction of the LDR and 10k goes to an analog pin on the Arduino.

The sensitivity is quite good, covering the full spectrum from total darkness to bright sunlight. With a bit of observation I was able to determine 'value ranges' to display in my home automation as "Total Darkness", "Moonlight", "Bright Moonlight", "Dusk/Dawn", "Gloomy", "Overcast", "Bright" and "Very Bright". If you read it often enough, you can detect a dip in the value as a cloud passes over. Other motion sensor nodes (in bath, kitchen and bedroom) send read requests to an indoor LDR node every 15 minutes to know if its dark enough to turn on that room's light when motion is detected. I never have to turn an lights on or off anymore. An external motion sensor is used to detect someone approaching the front door and turns on a light and sounds a small beeper inside, negating the need for a doorbell.

I also use one with my TV and computer screen to detect when the TV and/or my PC screen is on. This data is used to record the amount of time I spend watching TV or at my PC. The LDR is mounted in a small, 7mm black plastic cap like those used to protect the end of round tubing and is pointed towards the screen but shielded from ambient and overhead lights. It's simple, cheap and very effective.

You will find ready to use code for an LDR based node in the NodeManager library (link below).

Refer: https://github.com/mysensors/NodeManager

Hope the above helps...

Paul

Something like this one does:
http://diffractionlimited.com/product/portable-cloud-sensor/
"The sky temperature".

https://mynasadata.larc.nasa.gov/science_projects/measuring-the-temperature-of-the-sky-and-clouds/

Edit:
We could use MLX90614 sensor to measure the sky temperature and compare to ground temperature and in that way, detect cloudy or sunny days.
http://www.ebay.com/itm/MLX90614ESF-BAA-000-TU-ND-Infrared-Thermometer-Module-IR-Sensor-for-Arduino-/201402795181?hash=item2ee48ac4ad:g:Pl8AAOSw32lYorlr

or

http://www.ebay.com/itm/MLX90614ESF-BAA-000-TU-ND-Infrared-Thermometer-Module-IR-Sensor-for-Arduino-MO-/152234019326?hash=item2371dac9fe:g:fEYAAOSw9IpX0Qsl

Info about using with Arduino:
In this link we even have the correction factor and code that use the info from sensors:
https://indiduino.wordpress.com/2013/02/02/meteostation/

I will order a sensor and try this out! Should be able to port that to MySensors.