Which LED has the coldest color temperature?
Probably a blue/cyan LED will give infinite Kelvin.
the cheapest one, usually 
A blue LED will give you the highest kelvin in the visible spectrum 
uv 400-385 nmis usually a cheap uv led
it is usually fairly visible though
and you need a filter to make it useful
since a lot of things glow blue, if the light is blue also, you can;t really tell that
it usually needs to be 365nm or below, to be invisible
they can get below that but they get more expensive
wle
There are 10000K and higher nominally “white” LEDs for coral farms and the like.
Which are you asking:
LEDs with:
1) Visibly cool colour (bright white/blue colours), with high Correlated Colour Temperature (CCT) value of 7000K etc.
2) or visibly yellow/orange colour, which is a low CCT of 1800 K etc?
1) Visibly cold colors (bright white/blue colors), with a high correlated color temperature (CCT) value, 7000K, etc. Please call such lamps.
Great, thanks for clarifying! There are many options out there- you’ll get more helpful recommendations if you state your required output and format, plus the planned end use :+1: