Dun is what's known as a colour dilution gene - the D gene.
- D is dominant, and if present in a horse which is bay, black or chestnut lighten the base colour of the coat. Unlike the G/g grey gene this isn't 'going grey' - the horse's coat is lighter from birth, and it doesn't matter whether the horse has a DD or Dd pair.
- d means you get the standard bay/chestnut/black colour as per normal.
The other key aspect of a dun horse is what are known as 'primitive markings'. These are, in order of most to least common/visible:
- The 'eel stripe' - a black stripe along the centre of the back.
- Leg stripes - often faint horizontal striping on the back of the (fore)legs
- Shoulder stripes - much rarer
In addition, the face of a dun is often darker than the rest of its coat.
The three basic coat colours interact with DD or Dd as follows:
- Bay (AA or Aa, EE or Ee) produces a bay dun or 'classic' dun with a light brown coat and darker main and tail, and primitive markings.
- Black (aa, EE or Ee) produces a "blue dun" or "grullo", with a grey coat and darker markings, points and often face.
- Chestnut (aa, ee) produces a "red dun", with darker red points, markings and face.
A dun with the G (gray) gene will exhibit normal greying behaviour with age.
Roan, on the other hand, is not the product of a colour dilution gene, but it does produce an effect which can look similar at a distance.
- Rn (guess what, it's dominant) manifests itself as an even mix of white hairs and those of the original coat colour on the body, which (unlike a grey) does NOT lighten with age (and again, because it's a dominant trait, it doesn't matter whether your horse has an Rn/Rn or Rn/rn pair). Note that the head, mane, tail and lower legs remain dark.
- rn means you get the standard bay/chestnut/black colour as per normal.
In a similar way to a dun, the three base horse colours produce named variants:
- Bay + roan produces a bay roan
- Chestnut + roan produces a red roan, although the paler variants can be called a 'strawberry roan'
- Black + roan produces a blue roan.
The key difference at the kind of scale we're likely to be painting figures at is that a dun has the 'dun face', the often darker colouring about the face, and the primitive markings, whereas the roan's head, mane, tail and lower legs remain dark. Another difference is that if a roan's coat is damaged (by, for example, a cut or a brand), it tends to grow back in the base colour without the white hairs.
So, that's duns and roans. Next, what are variously called 'paint' or 'coloured' horses.