Is ballast a uniform colour?

Ballast is not really a uniform colour.

Although it would have started off the same colour on a prototype line, over time the ballast would be susceptible to everyday dirt and grime from the locos.

Here is an example taken at Havenstreet Steam Railway (Isle of Wight) of a section of track next to a water tower you will see that next to the crossing gate in the middle of the picture the ballast is brown but nearer to the water tower it is black from the continual stops made by locos to fill up with water.

This would be very similar at the coaling stage area or anywhere the loco stops for any period of time.

Why not check out your local preservation railway and take a look?

Take some photographs for reference and have a great day out at the same time.