A topic dear to most modellers hearts  :twisted:

In the past I have used the same colour ballast on the whole layout but I was wondering what happens in the real world where the ballast is sourced from different parts of the country ?   I understand that different types of ballast on the same track are common in some parts of the US but what about the UK ?
Does it simply change when they run out of one type and start using the other?   Are there any real life examples of ballast colour change?

Yes, the ballast colour changes from time to time and place to place in the UK and in Australia too.

It also changes dramatically where new ballast is laid for a stretch of line but meets much older ballast at either end. Old ballast is stained by oil, grease and brake dust, as well as blown dust or soil from the surrounding environs. Ballast in station areas suffers a lot more oil and grease staining where trains stand.

Ballast colour can vary on adjacent tracks too, with one having been laid years before the other, for example, or one having been renewed more recently.

It can be a good idea to mix ballast colours on your model railway; I like to lay a bit of grey ballast then run strips of fine brown ballast along the length between the rails (the 'four foot'). 

Ballast changes are not as common as they once were. The Line I'm modeling in EM gauge Started off with chalk as ballast, then moved to a yellow limestone when the chalk turned to mush. When GWR took over they started using grey granite, For most of England that is the common ballast. However Scotland was famous for red granite ballast, in some areas.

My N gauge layout was started by the original builder with red ballast, but a lot was later added in Grey, which for the moment I'm using in the rebuild..

On some railways they used iron ore slag as ballast, all sorts of colours can be in that when you look closely.

On a modern layout you won't be far out with grey, then weathered.

Meldon ballast quarried on the fringes of Dartmoor is pale grey when first used in contrast to Shap ballast which is distinctly red.  As Jeff says they weather down but will usually still be distinguishable even when seriously grimy.  Even within a quarry there can be variations in colour since rock being a natural substance is not usually one even shade throughout.  

For modelling purposes I mix colours.  Woodland Scenics medium buff 3:1 with Medium Brown.  I also weather it after it has been laid and firmly glued down.  That involved the application of powders and / or acrylic washes.