UNDERTINT
colour mixing for watercolourists
Mix the colour in front of you
Undertint works out the recipe. You paint.
How Undertint works
Beta version shown.
You tell Undertint the specific brands and colour you own and it builds all its mixing recipes from that palette. If you’re painting with Winsor & Newton, the recipes are derived from those Winsor & Newton paints. Switch to Holbein and the recipes change, because the pigments are different and so is the physics.
Pick a colour from your screen and Undertint gives you a mixing recipe you can actually use. Not a generic approximation but from the paints on your desk.
Photo simplification
Import any reference photo and Undertint reduces it to the colours that actually matter — the ones you’ll mix from your paints. It uses perceptual clustering, which means it groups colours the way your eye does, not the way a computer would.
Your palette, not a generic one
Recipes are built from the paints you own. Search across paints from the brands you use, add what’s on your desk, and every recipe Undertint gives you will come from that palette.
Mixing recipes that work on paper
Each recipe shows paint names, pigment codes, and approximate ratios. It also shows a dilution gradient with your target marked, and modelled spectrally rather than as a simple fade to white.
Tonal structure before you touch paint
Switch to value mode and see your image broken-down into tonal bands with a proportional bar showing how much of the composition each zone occupies.
Grid overlay and detail zoom
A configurable grid sits over the simplified image to help with compositional planning. Double-tap any cell to see that region at finer detail, useful for faces, focal points, or anywhere the broad simplification loses something.
Why the recipes actually work
Most colour tools think in light. They work in red, green, blue which mirrors the way screens produce colour: by mixing wavelengths. Mix red light and green light and you get yellow. That’s fine for a display. It’s useless for paint.
Pigments work differently. They absorb light rather than emit it. The colour you see is what’s left after the pigment has taken what it wants from the spectrum. Mix two pigments and you’re combining two different patterns of absorption. This is why mixing paint colours is so much harder to predict than mixing coloured light, and why RGB-based tools give you recipes that look right on screen but wrong on paper.
Undertint uses the same colour mixing theory your paint manufacturer uses, and models pigments the way pigments actually behave: as light-absorbing materials with measurable properties. Those properties vary by manufacturer. Winsor & Newton’s French Ultramarine (PB29) absorbs light slightly differently to Holbein’s, and Undertint knows this. The recipes that Undertint gives you are specific to the paints you’ve told it you own — which is the only way a recipe can reliably work.