Deontology is a way of thinking about value which can be most easily introduced by contrast with the idea that the value of a thing depends on its consequences. Many versions of deontology emphasize rules, which must be followed whatever the consequences. For example one must keep a promise, punish the guilty or tell the truth even if it leads to bad results. Consequentialists deny this and say that one should sometimes break a promise, punish the innocent and lie if it has better results. Often the contrast is that the deontologist says that duties are imposed by the past not the future. The fact that something happened in the past, such as the fact that I promised or the fact that a crime has been committed, casts a normative straight-jacket over the present. But, at its most basic, the deontological idea is that the value of a thing does not depend on extraneous matters. There is a positive value that is intrinsic or internal to a promise being kept and a crime being punished and a negative value intrinsic or internal to promise-breaking, punishing the innocent and lying. It is only what exists (the Greek 'onto'), there in the act, which determines the value, and not matters external to it. This is a fundamentally austere view of value, something that inspires awe in some and accusations of inhumanity by others.
Now a crucial part of this view, which prevents such value from seeming fickle or fleeting is that once a value is there it imposes an inescapable demand for consistency. If one case is good or beautiful or just then any other cases relevantly similar must have the same value. This gives deontology practical bite. If a killing is bad on Tuesday then a similar one done on Thursday must be bad too. If you were wrong to take a bribe then I too was wrong when I took a similar bribe. (We cannot make exceptions in our own case.) Keith Coventry's Ten Deontological paintings exemplify or illustrate or symbolize this iron demand for consistency. The ten paintings have differences but they are differences that make no difference to their value. They differ in texture. But those differences do not make a value difference. They also differ in their history. Some were made in London and some in China. But that difference is not a difference that makes a value difference. (Clive Bell claimed that it makes no difference whether a sculpture was made last year in Paris or 5000 years ago in Africa.) Coventry's ten paintings sit there saying, "One for all and all for one". Or rather, "Whatever value one of us has we all have."
Furthermore, the paintings' greyness is a celebration the austerity of the deontological vision of value. Some see morality has a human device, something to help us live together and to be happy. This is a cosy nice comforting human vision. But it is not inspiring. It is nothing to die for. It could inspire no heroism. The pure almost abstract nature of the demand of duty, which with mathematical necessity inexorably bearing down on me, indeed on all of us... in what colours might we represent that? The deontological demand is not our friend. It is something before which we must submit, we must fear the norm. There is after all no escape from it. But in its very purity lies its steel strength and its deep psychological appeal to that part of us that aspires to more than comfort and happiness. The very harsh coldness of duty, in its very lack of superficial incentives, can inspire us to fit ourselves to the demands of duty rather than tailor for ourselves an undemanding lazy vision of how we should be. There is beauty in the austerity of duty.