Water clear ultraviolet LED 5mm gave 2.4Volt on 1Meg resistor (photo voltage, voltmeter's R=10Meg), clean sky (June24th, at high noon). The other colours (all water clear 5mm) did between 1.5V - 2.0V.
PS:
uv 2.36V
white 2.12V
yellow 1.69V
red 1.51V
green 0.02V ??
It seems the Sun is not green enough.. 
So with an high impedance buffer (an opamp) you are exactly in the ADC range. Mind the LED's radiation diagrams might be VERY narrow, so it was not easy to find the maximum voltage while measuring..