A screen colour is not a pigment

Why RGB mixing lies

Add blue light to yellow light and you get white. Mix Ultramarine into Cadmium Yellow and you get green. Screen colour is additive, paint is subtractive, and any tool that averages hex values will confidently hand you recipes that cannot exist.

Mixed the way pigment mixes

Each tube’s colour is converted into its Kubelka–Munk absorption and scattering behaviour, mixed in that domain by weight, and converted back. Blue and yellow make green. White lightens by scattering rather than by dilution. Earth colours dull chroma the way they really do on a palette.

Ratios a painter recognises

A light skin tint is not three parts white to one part sienna — it is closer to forty-eight to one, a knife-tip of colour into a pile of white. Recipes are searched across a wide geometric range of ratios for exactly that reason, and reported both as parts and as proportions, with “a touch” for anything under two per cent.

Your tubes, not a generic set

Tick what you own from a library of common artist oils, correct any swatch that does not match your tube, and add brands that are missing. Every recipe is then computed only from those, so nothing suggests a colour you would have to go out and buy.

How close is close

Each recipe carries its CIEDE2000 difference from the target. Under about 2.5 is a very close match; 3 to 6 is a good working match for painting. On a nine-tube starter palette, typical portrait colours land around 1.5 on average.

The one honest limit

No mixture can be darker than your darkest tube. A near-black target may stop around a difference of 3 with Ivory Black on the palette, and no amount of searching will improve it. That is the paint talking, not the software.