Kelvin is not a linear scale. Every correction is.

Colour temperature is quoted in Kelvin and corrected in mireds, where a mired is 1,000,000 ÷ K. A filter is a constant shift in mireds — that is what lets gels stack by adding, and it is what makes Kelvin arithmetic wrong.

The engine is free and needs no account. A full CTO is +131 mired wherever it is used. On 5,600 K it lands at 3,230 K — a change of 2,370 K. The same sheet of gel on 3,200 K only reaches 2,250 K, a change of 950 K. Same gel, two and a half times the Kelvin change, so every rule of thumb phrased in degrees is true at exactly one starting point. Mixing follows the same rule: sources mix by a level-weighted average of their mireds, so half a 2,700 K practical and half a 6,500 K sky is 3,820 K, not the 4,600 K everybody does in their head — 780 K out, and the error grows with how mixed the light actually is. Every lane ships a worked example that costs nothing. Writing the report needs credits.