"...I've seen the photos; it's not red..."
The photos both of the cellar floor, and of the stuff in the boot tread, that were shown on the program, looked pretty red to me. Now, those photos might have been taken with a filter to enhance the color, but I'm describing them as I saw them. If you are familiar with the notions of hue, value, and chroma, in quantifying color, the color appeared to be somewhere between a 2.5YR 5/8 and a 5YR 4/6. That's a lot redder than "tan".
You're talking about the shadows and dark brown part, then? It does have a reddish tint to it, but the actual bootprint is in a light tan area.
When you look at the whole room, there is more concrete gray-colored, white or light gray, growth than anything else.