Figured it out, or at least a workaround. In my version of the UE (4.5) you can’t change the text Colour and Opacity parameters dynamically, by binding a function that does this. However, you can set its opacity to 0 and change the Shadow Colour parameters using a function.
Alternatively, if you still want to have the text and the shadow, you can do this by adding two different Text Blocks and set one of them to Visible (in Details>Behaviour) and the other to Collapsed (it makes the text invisible and not to take space). You can change the visibility by binding a function. Set the different colours you want and then just swap the texts whenever you need.
I am new to Unreal Engine 4 and game making in general so there might be a smarter way to do this, apologies for misleading anyone.
Hopefully this is changed in the next versions of the engine; for some reason I failed to install 4.6.