Welcome to Harry Potter Theory. Today we're discussing spells. Specifically, the VISUAL aspects of spells.. Now, there are all sorts of spells and all kinds of different incantations, but one thing is for certain, the colour of a spell always attracts a lot of attention. Voldemort’s avada kedavra- just wouldn’t be the same without that sinister looking jet of green light emanating from his wand, and nor would Harry’s bright red expelliarmus. 
But, what I want to discuss today is- what do these different spell colours actually mean? Are they simply arbitrary? Or are we able to derive some sort of a relationship between the colour of the spell and the type of spell itself. What is the underlying meaning behind these spell colours? 
I think that the most obvious and sort of go-to answer to this relatively unaddressed question is something like: green spells are inherently dark in nature, red spells are inherently powerful in nature, and white spells are inherently pure, good, and passive in nature, to name a few. 
And I agree with those sentiments, for the most part. However, I believe that there is much more to it than that, and I want to peel back the layers and uncover what it is that makes spells these colours. Let’s start by looking at a couple of common spell colours.