?Viewsheds produce results of either red or blue. Is there a way to change this default or do I need to change the colours after processing? How do I change those colours to specific ones?
You can't change the default.
Viewshed produces either a "continuous" field raster (with values of 0 or 1) or a "classified" field raster (with classes of invisible or visible) as output.
For "continuous" rasters you need to use pseudocolor rendering - meaning you need to set up a color table and a data transform. As there are only two values in the raster only the first and last colors in the color table will get used. I often turn on "clip limits" and make clipped values "transparent" and make the minimum and maximum limits both equal to 1. This makes all the cells where the tower can't be sighted transparent so you only see the positive results.
What you can do is save a rendering "template" and then just apply this every time you generate a new viewshed and want to render it. You do this from the "Favorite Templates" button on the raster ribbon. I find that is convenient.
For "classified" field rasters the color of the invisible and visible classes is baked into the file and this is hard-coded. However, you can change these colors via the "Advanced color" dialog when rendering the raster - just click on them and you can edit. This will modify the raster permanently.
Classified as a default setting would be good. Now I have found it, life is much easier 😃 ?