Obviously once you reach green you know the basics. You have general map knowledge of where each 'type' of tank should go and you know basic tactics such as sidescraping and going hull down. So from green up to unicum it's *mostly* just micro improvements, specifically in the positioning of your tank. If there's one main difference I can pick out between a green, blue and purple player, it's the transition from playing predominantly well armoured, hard hitting TDs & HTs, to fast, reliable MTs.
Purple players (generally) love flexibility, and good gun handling. This is because it allows them to go TO the damage rather than waiting for it to come to them, and once they're in position to do the damage, to do it reliably. Less experienced players tend to underestimate value of flexibility and relocating, instead preferring just raw armour and firepower. To me at least, it seems that less experienced players don't like fast tanks with no armour for the same reason kids don't like coffee, they just haven't developed the correct taste for it yet. So I would recommend that if you want to improve, develop the taste for flexibility. Try to always be REPOSITIONING for better shots, not waiting for them.
One of the best ways to improve in my opinion is to simply watch what good players do. Most of the good positions I've learned have come from seeing someone else use it in game, then next time I try it out for myself. So if you don't have xvm installed, I'd recommend installing it just so that you can see who the good players are and where they go. Another way is to simply watch good players play on twitch. I personally have learned a lot from watching skill4ltu's stream, he is arguably one of the best players in the whole game, is entertaining to watch, and actively responds to questions in the chat. But whoever you choose to watch, as they're playing be constantly asking yourself "what would I do here?" and compare it to what they did, so you can understand in real time exactly what they're doing better than you.
Other than that all I can say is just try to stay alive at all costs. You might sometimes feel obligated to stick with allies on a collapsing flank, but just remember you're more valuable to your team alive with a chance rather than dead with the people who weren't smart enough to run with you.