Colour shift paint contains flakes that reflect different wavelengths depending on the angle you view them from, so a surface painted with it reads as one colour head-on and another at a glance. On a miniature, which is a small object turned constantly in the hand and seen from many angles across a table, that effect is far more visible than it is on a flat surface.
How the effect works
The paint carries interference pigments, flakes engineered so that light reflecting off their front and back surfaces interferes and cancels some wavelengths. Change the angle and the path length changes, so a different set of wavelengths cancels and the apparent colour shifts. Because the effect depends on flakes lying flat and being seen against something, these paints are translucent and their appearance is decided as much by what is underneath as by the paint itself. That is the single most important practical fact about them.
The undercoat decides everything
Over black, colour shift paints look their most dramatic, because the dark ground absorbs the light the flakes do not reflect and the shift reads cleanly. Over lighter grounds the effect washes out and the paint can look like a slightly pearlescent version of one colour. Most painters using these paints prime black or apply a black base coat first for exactly this reason, and a first attempt over grey primer is the usual cause of someone concluding the paint does not work as advertised.
Where it belongs on a model
Because the effect needs an angle to change, it is most convincing on curved and faceted surfaces: carapaces, helmets, vehicle panels, wings and cloaks. On small flat details it is invisible, because the viewing angle never varies enough. Used across a whole model it tends to look like a novelty; used on one large surface against ordinary paint elsewhere it looks deliberate. It is also worth remembering that a matt varnish will scatter the light the effect depends on and largely kill it, so a gloss or satin finish is usually the right ending.
Questions people ask about colour shift paint
What is colour shift paint?
Paint carrying interference flakes that reflect different wavelengths at different angles, so the surface appears to change colour as the model is turned.
What undercoat does colour shift paint need?
Black, or a very dark base. Over lighter grounds the effect washes out, which is the usual reason a first attempt disappoints.
Can I varnish over colour shift paint?
Use gloss or satin. A matt varnish scatters the light the effect depends on and largely removes the shift.