There is a palette disaster every oil painter knows. You want a pale duck-egg blue. You put out a decent pile of phthalo blue, since it is the blue you need, and start adding white to lighten it. You add more white. You add a lot more white. Ten minutes later you have a tennis ball of paint that is still, unmistakably, phthalo blue, you have used half a tube of titanium white, and the duck-egg colour is as far away as when you started.
Your colour sense was fine. The order the paints went in was the problem. The sequence in which paints go into a mix is real craft knowledge, the kind that used to pass from teacher to student in ateliers and now mostly gets learned by ruining white. It comes down to one property of pigments that tube labels don’t advertise.
Tinting strength: pigments are not equal
Every pigment has a tinting strength: how forcefully it asserts itself in a mixture. It is a physical property (how strongly the pigment absorbs light relative to scattering it), and the differences between pigments are large. The modern synthetic-organic pigments, phthalo blue and green, quinacridones, dioxazine violet, Prussian blue, alizarin crimson, are very strong. The classical earths, ochres, siennas, umbers and terre verte, are gentle. In the pigment data behind Halftint’s engine, phthalo green is over twice the strength of an average tube and the earths sit below average.
The practical consequence is lopsided. A speck of phthalo will visibly shift a whole pile of white. A pile of phthalo will swallow a knife of white without noticing. Equal volumes do not get an equal say in a mix, and a strong tinter in quantity outweighs everything else on the palette.
Once you see that, the classic disaster explains itself. Starting from the phthalo pile means the weaker pigment (white is moderate at best) is trying to overpower the strong one by volume, and the volume it takes is enormous. You were not mixing badly, you were fighting the pigment’s strength with sheer volume.
The rule, and the reasoning under it
Start with the weakest, lightest, most plentiful ingredient. Add the strongest tinter last, in the smallest increments you can manage. A knife-tip. A speck dragged off the edge of the pile. For dioxazine or phthalo going into a pale tint, touching the knife to the paint and then to the mix is the actual gesture.
There are three reasons this direction works better:
Small additions are correctable and overshoots are expensive. If the mix isn’t blue enough, another speck costs nothing. If it is too blue, the only way back is to mix a fresh batch of the base and dilute the error, which costs white, time, and usually a mix that drifts chalky. Approaching the target from the weak side makes every correction cheap.
You are usually mixing towards a lighter value than the strong tinter. Most target colours on most paintings (skies, flesh half-tints, greyed greens, anything in light) sit far up the value scale, close to where the white and the earths already live. Starting there means the bulk of your mix is already about right and the strong pigment only has to steer.
The first pile sets the batch size. Start with white and you end with roughly the amount of pale blue you wanted. Start with phthalo and the batch grows with every rescue addition until you are mixing enough duck-egg to paint a duck pond.
Which of your tubes need this care
You can rank your own rack in five minutes with a tint test: a small equal part of each colour into an equal part of white, side by side. Treat these as strong until proven otherwise: phthalo blue and phthalo green, Prussian blue, alizarin crimson, quinacridones and the permanent rose and magenta family, dioxazine and most mauves, and, which surprises people, ivory and lamp black. That last one is why “just a little black to grey it off” so often kills a mixture. Ultramarine sits in the middle: assertive but manageable. The earths you can add by the knife-load. Every brand chart on this site flags its strong tinters for this reason.
The same ranking tells you how to sequence a three- or four-paint mix: weakest and lightest first, mid-strength colours next, strongest last. For a greyed sky blue from white, yellow ochre, ultramarine and a touch of alizarin: white pile, ochre into it, ultramarine in small knife-tips to colour, and the alizarin last as single specks. Never the reverse.
Two housekeeping habits help. Wipe the knife between the strong pile and the mix. The residue on a knife that has just cut phthalo is a meaningful dose for a pale tint. And judge against the painting, not the palette. Pull a small smear to the canvas edge before committing the batch.
Where the maths meets the knife
This is also why mixing software has to state proportions in a form a painter can carry out. When Halftint computes a recipe, the engine knows each paint’s tinting strength, so it doesn’t just say what to combine. It gives the order, in whole knife-parts, and flags the strong ingredient: add the phthalo last, a speck at a time. The proportions are approximations, and the ΔE is shown on every recipe because pigment mixing is approximate by nature. The sequencing advice is the atelier rule above, applied to your tubes. When a mix has already run away from you, Mix Fix will compute what to add to steer it back, or say that it is headed for mud and a restart will cost less than the rescue.
The rule itself needs no software and pays back in white within a week: weak and light goes down first; strong goes in last, a speck at a time. It is worth pinning to the easel.