Hi Sabstar ,
your colour is based on the expression you have defined, as all three expressions fulfil the red colouring condition that is what you have displaying in your legend.
You might want to think about having a set colour for each expression and then look to adjust the line style instead for those weekend days, to show your difference. Solid for working day, dashed for weekend for example
hope that helps
I have not tested this, but you might be able to use Dimensionality() to conditionally colour the lines. In your foreground colour, expression:
=If(Dimensionality()>0, ... existing colour expression ...)
Insert your existing expression into the above. This should prevent the legend from being affected by the colour expression.