Mixing colours and pothole formations

If all colours mixed make black, as is true with light, how come, when mixing paint or, indeed, coloured plasticine, you end up with a mucky brown/grey colour?

The questioner is confused — all the colours mixed (the spectrum) in light is perceived as white — hence Newton’s discovery of what composed sunlight when refracted through a prism. What she is talking about is what happens when one takes a solid dye such as paint or ink or plasticene.

The colour of anything — be it paint on a palette, the breast of a robin or a picture on the page of The Times — is determined by what parts of the spectrum are reflected back. So the red of a robin’s breast appears red to us because the red and yellow waves are reflected back by the pigment in those feathers, rather than being absorbed. Anything that appears black is reflecting nothing back — when we perceive a black door, we are literally seeing nothing colourwise.

As regards paint, to make black, you need a mixture of 100 per cent cyan (C), 100 per cent magenta (M) and 100 per cent yellow (Y). This makes black, called in the trade “registration”.

You can see registration marks every day in newspapers — they are those little dots that run up the side of outside edge of a page. In some papers, they run them along “gutter” — the inside edge of the page — or along the bottom or even the top: anywhere outside the main print area.

The registration marks are useful for printers, because they let them know at a glance if one of the colour plates on the printing press is not aligned properly — the C or the Y or the M dot, or a combination or them, will be out of line with the others (readers will also have to strain their eyes to see any colour photos properly).
— John Symes, production editor, Newsquest South, London SW19

Potholes should occur at the outer parts of the road, at the points of maximal stress, resulting from wear and tear by the wheels of vehicles. Why then do potholes have a random distribution?

I am not sure that stress is the main factor in the development of a pothole. More significant seems to be places where water can penetrate the tarmac and subsequently freeze, loosening the structure, eg, places where road repairs have left an imperfect junction with earlier tarmac, places where a tree persistently drips, even places where there seems to be a below-surface spring that defies the repairers year after year.
— J. R. Pope, Tisbury, Wilts

Who is the oldest actor to have played Hamlet on the West End stage?

May I nominate Sir Arthur Palgrove? “He went on playing Hamlet till he was 68. There were more lines on his face than steps to the gallery.” (Ronald Harwood, The Dresser).
— Michael Gray, Chelmsford

On cloudless, cold mornings, why are aircraft vapour trails short-lived on some days and long-lived on others?

Terence Chubb is wrong about water vapour being absorbed into the air (April 18): that’s the “windbag argument” disproved by John Dalton in 1802. Water vapour behaves independently of the other atmospheric gases; indeed, it behaves just the same in a vacuum. The duration of contrails depends only on the temperature of the ambient air; the lower the temperature, the longer they last.
— Steuart Campbell, Edinburgh

How do woodpeckers avoid concussing themselves when they obsessively hammer away at tree trunks?
— Robert Randell, London SE26

What is the name for an inhabitant of Argentina? Sports commentators dither between calling players Argentines or Argentinians, eg, Andrés Romero is an Argentinian golfer, but David Nalbandian is an Argentine tennis player.
— Elizabeth McIntosh, Dundee

When and why was the practice of jumping over the net by male winners of tennis championships stopped?
— John Peters, London SE5

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