Ok yes I see now. x8 = 3 stops so x6 = 2.5 stops making x5 = 2.25? Stupid maths! So there really is very little difference in the filter factor between the orange and the red. There can't be much harm in calling it three stops and slightly overexposing like I usually do though can there? I need to get this down to a science which will hopefully come with the 5x4 I've bought. There's a discipline I'm lacking. Perhaps I should've gone for a deeper red (x8). The bloke had one and it only sold for £6. Is there going to be very much of a marked difference in the effects caused between them. Ta Rob thanks for putting me straight.
correct there would be no more than 1/4 stop difference between the 5X and 6X filters.
Using filters is not quite that straightforward because the effect they have depends very much on the subject colour. Firstly, for black and white subjects you will find that anything not in direct sunlight will usually have a lot of blue in it and so yellow, orange and red will all darken shadow areas. Red filters have a very dramatic effect. So much so that many people find it too strong a filter for "normal" usage whereas a light yellow filter will have a more subtle effect. You have to learn how each filter will affect the result based on experience and careful judgement of each subjects colours.
For example, just throwing a red filter on will not necessarily make the sky go black. The reason is that when you look at the sky, especially the nearer you get to the horizon, the sky tends to get very much lighter and contain a lot more humidity/haze which is frequently not removed by filters. Maybe partially but not necessarily fully. On the other hand if you are at high altitude where the air is below freezing so all the humidity has frozen out of the atmosphere and the air is really dry, then the sky becomes a much deeper blue and a light yellow filter can make the sky completely black.
Alternatively you might photograph a red flower using a red filter. Place the flower on say zone 7 or 8. Result is that flower comes out almost white and really stands out against anything not red behind it.
So it really depends on what you are trying to do and your subject. Trial and error is the name of the game to find out what filter factor works for which subjects and how you can affect the contrast of different coloured subjects.