I’ve been wondering if warm white LEDs are viable for use, especially the 3000K and 2700K ones. Amber LEDs are generally used less often outside of wildlife situations due to their lower efficiency while 4000K LEDs are widespread in many authorities. I only know a few authorities around me that utilize 3000K or 2700K LEDs, therefore are they as efficient as 4000K LEDs or slightly less?

You have answered your own question. Warm white LEDs are in use, so they are viable. Wiltshire uses them for example, the standard being 3500K but 3000K are also used in heritage areas.

White LED light is usually generated by blue or UV LEDs - which are the most efficient types, together with a phosphor (think MBF lamp or fluorescent tube) to produce the white light. White light can also be produced by RGB LEDs but the red and green are significantly less efficient, hence why phosphor conversion is the most widely used approach.

As LEDs have become more efficient this has offset the phosphor conversion loss and hence the move to warmer colours compared to earlier generations of lanterns. Warmer colours are more visually appealing, although the eye is more sensitive to the green blue part of the spectrum, than the red.

The original post seems like a non-question. Warm white LED is commonplace and widespread, therefore it is clearly viable.