I'm totally wild guessing here:

>! Scarlet

Because:

>! It seems to me that by adjacent we should consider words that are "connected" via another word. That is, not the words x and x+1, but x-1 and x+1. So for example we pair "letter" and "number" and get "symbol"

So, we get a partial solution for some of the words:

>! **scarlet**<br/>fruit - A (scarlet letter)<br/>letter - apple<br/>computer - symbol<br/>number<br/>subject - consumer<br/>consume - history<br/>before - <br/>cut - happy<br/>cry - <br/>slide - weep<br/>gentle -<br/>awake - rought<br/>not - <br/>rain - hot<br/>cold - 

We basically need to find a color:

>! the color must fit with rain and letter. So I'm guessing **scarlet** for: "scarlet rain" (because of [this](http://en.wikipedia.org/wiki/Blood_rain) or perhaps blood in general) and the [The Scarlet Letter](https://en.wikipedia.org/wiki/The_Scarlet_Letter), which is the letter "A".