Of all the Parker 51 colours that can cause confusion the 'reds' are probably the most troublesome. I include the burgundy colours as red.
The photographs are taken in natural light and havn't been altered in any way so the comparison is as accurate as possible.
From top to bottom the colours are: American Burgundy, English Burgundy, Rage Red, and the bottom pencil is also Rage Red but is a Parker 61 not a 51.
1.The American Burgundy looks decidedly brown. It is often described as Cordovan Brown but this is not carrect as the Cordovan Brown was only used on the Vacumatic pens, all these are Aerometrics.
2.The English Burgundy is much 'redder' than the american version, the Americans often call it 'Blood Red', or 'Bloody Burgundy', the description doesn't have the same vulgar connotations as it does in England.
3. The Rage Red was only used on the Mk. lll and Mk. lV pens made from 1969. They are easily recognisable by the sqared off end to the barrel
4. This Rage Red is even brighter, it has an orange/pink cast and was only used on the Parker 61. It doesn't really belong here but it was wort including as both colours have the same name.
Here is a close up of the three Parker 51 'reds':
So, despite the confusion it is clear that:
If a Parker 51 looks brown, and it is an aerometric pen then it is American Burgundy.
If it looks red/ maroon the it is English Burgundy.
If it has a squared off end and looks bright red the it is a Rage Red Mk.lll or a Mk. lV pen. The Mk.lll has a pearl coloured cap stud and the Mk. lV has a metal stud.