How have the HEP bikes got yellow tanks?

Paint?

Sticker or vinyl wrap?

BarX is also running yellow tanks. I love the look of the Suzukis this year. 

Acerbis (and probably others) have

made them since 2018... I've had black ones since about then.

https://www.cyclegear.com/parts/acerbis-gas-tank-cover-suzuki-rmz-450-2…

The top of the tank has a small plastic cover, stock is blue but aftermarket companies sell them in a handful of colors.

Edit: just saw this was already answered above, sorry for the additional reply!

 
Not the best picture but yes just a cover, that stock blue is so ugly and so is the seat 😬

When I bought my RMZ it had a blue tank cover. And blue side plates & fork guards. First pic you can see how it bolts on. 

I swapped it for black 😁

 I already got my 2023 skda twisted tea graphics, just need plastics now.

2024 rmz are gonna be all yellow, you read it here!😊

So it's just a plastic cover for the tank?

Does it serve an actual purpose besides changing colors?

Correct, just a plastic piece that goes on top of the black plastic oem tank.  Even the blue that you see on the stock bike is a blue plastic cover.  UFO makes them in yellow, white, black, flo yellow, and oem blue, as that is what the JGR team ran.  Acerbis I would assume offers them also in different colors.  All of the raw oem RMZ tanks are just black plastic underneath. 

It holds the front of the seat down so it does serve a purpose.

I had the yellow cover on mine in 2020.

Your bike looks good!  I am a little partial to it though, as I kind of know the fella who maybe made your graphics  The 2020 SX season started with JGR running white rear fenders and side plates, but after only a few rounds there was trouble getting the white rear fenders and having enough to last the season, so the team switched to solid yellow plastics.  All except for the fork guards..they stayed oem and stayed black.  The oem fork guards are thicker than the aftermarket, and less prone to having holeshot devices rip out.  Just a little tidbit there after seeing your bike.
 

I do remember making yellow and black decals for the tank covers early on before UFO could send different colored plastic ones.  Forgot about that.   

I’ve been loving the yellow with gold rims. It’s fresh. 

 Yellow cover looks good! CMT Carbon is nice too! The purpose is that you can change all the colours of plastics, going full yellow, full black, full white etc

You are going to need a bigger scale for the rest of the bike. 

I try to make my bikes lighter, that's why I use balance scale.

That's my last build. RM 144 AF