How can I know the colour change of this reaction?
Potassium dichromate (VI) is the species that turns from orange and to green when reduced. Most of the organic substances I came across for testing purposes are colourless, so I would assume that compound B would be as well.

It is just like you are expected to know the colour change of potassium manganate (VII) from purple to colourless when it is reduced.

Those two oxidants were used frequently in my labs, and I'm pretty sure you'll be using them often as well.

I hope this answers your question! :)