Fibres dyed with natural dyes can change colour, depending on the pH level of the liquid in which is was dyed.
For example, this red nuno felt shawl is dyed with brazilwood after using a mordant of alum and cream of tartar, which makes the dyebath more acid. It creates a beautiful bright red. If I would put it into a bath that is even more acid, the colour would shift to orange. If I would add an alkaline modifier, such as baking soda for example, the colour turns more towards maroon. These changes are very exciting when we like them to occur.
But what if later on, we want to wash a felted scarf and we don't know whether the colour is going to be changed by the pH of the soap solution that we will be using for the washing? Very simple, either, wash it in just water or use a washing liquid that has a neutral pH, this means, pH 7.
To help you a bit, most dishwashing liquids are more or less neutral. Use a tiny bit of it in your washing water. DO NOT add lemon juice or vinegar to your washing water. If you are washing wool, felt included, make sure to treat it gentle and avoid moving and stirring.
Some colours are not at all sensitive to pH changes, such as indigo blue.