Tools
Color temperature visualizer
Kelvin numbers mean very little until you see them. Move the slider and the panel shows what that color temperature actually looks like.
Color temperature describes whether light reads warm and yellow or cool and blue. It is measured in Kelvin, and the scale runs backwards from what instinct suggests: lower numbers are warmer, higher numbers are cooler. 2700K is the familiar incandescent glow, 4000K is the neutral white of an office, 5000K and above starts to read like daylight or a hospital corridor.
The single most common mistake is mixing temperatures in one space. A 3000K downlight next to a 2700K pendant looks like a mistake even to people who could not name what is wrong, because the eye is very good at comparing two whites side by side and very bad at judging one in isolation. Pick a temperature for the room, and keep it consistent across any sightline where two sources are visible at once.
Color rendering matters just as much and gets far less attention. CRI measures how faithfully a light source shows color compared with daylight, on a scale to 100. An 80 CRI lamp is acceptable in a garage and disappointing over a dining table. Anywhere finishes, artwork, food or skin tones are on show, specify 90 CRI or better and accept the small efficiency penalty.
Before you rely on it
This is arithmetic, not a photometric calculation. It will get you to a sensible starting point and tell you when something is obviously wrong, but a real layout gets run in AGi32 against the actual IES files. Send me the plan and I will run it properly.