Pale crag martin to do so, it would cast doubts on their specific distinctness. There are several subspecies differing in plumage shade or size, although the differences are clinal, and races interbreed where their ranges meet. The pale crag martin of the nominate subspecies "P. o. obsoleta" is 12–13 cm (4.7–5.1 in) long with light brown upperparts, becoming paler on the lower back, and a short square tail that has small white patches near the tips of all but the central and outermost pairs of feathers. It has a pale grey throat, upper breast and underwing coverts, and the rest of the underparts
The Missing Shade of Blue people to recognise that there is a missing shade than it is for them to actually form a clear idea of that missing shade. Be that as it may, this dissolving of the problem fails to meet the second and third criteria listed above. Mental mixing is the solution proposed by Morris. The idea here is that just as paints are mixed to produce the range of colour swatches found in a hardware store, so it should be possible for colours to be mixed in the mind in some kind of analogous way. However, without further argument it is not