Noticed colourful 'graffiti' on sheep? Why? For many years (and in some places still) sheep were grazed on common land, which belonged to the whole community so different flocks mixed together. Farmers used paint marks to differentiate their sheep from others. It was originally a combination of pigment and grease so the colour stayed on. Chemical sprays are now more commonly used.
Each farmer would devise their own shape or combination of stripes. Eventually local manuals of smit marks were published, showing which farmer used which marks.
In addition to these ownership marks, you may see other ones. After scanning, farmers may mark ewes to show how many lambs each one expects, or mark new born lambs so it’s easy to know which is their mother.
In the Autumn/Winter you’ll see colour on the backsides of ewes. Tups (rams) are fitted with a raddle on their chest which contains colour. Each time they tup (mate with) a ewe, some of the colour comes off so the farmer knows which sheep are likely to have lambs.