really cool idea but i think your icon looks a bit synthetic
i tried it myself and it's indeed hard to find the right coloring
here's what my results look like:
https://www.dropbox.com/s/m15hueie12xclsy/flamethrower_ammo_graphics.zip?raw=1
you also need to change the color the fluids have in pipes and tanks
for the scarlet one i used:
base_color = {r=0.7, g=0.25, b=0.3},
flow_color = {r=0.7, g=0.3, b=0.35},
for the orange one i used:
base_color = {r=0.8, g=0.25, b=0.03},
flow_color = {r=0.85, g=0.35, b=0.06},

but if you don't use scarlet you should also replace the icon for the flamethrower ammo

Yeah, I was actually redoing the colors yesterday; dark and brownish may be realistic but it's not gamey, flamey, or intuitive.

Today I added some code to let future/modded flame turrets use it too.

i don't know if that one is better :P

Well, I like it...
In any case the flow colors have more of a "Super Mario magma" feel now which I think is much better.

to add my 2 cents, while i agree that the image in the preview image looks very "unprofessional"/fast&dirty made i like its overal style more than those "just another plain" color suggestions (while - if i had to choose one of them - i would definitivley pick orange, scarlet doesn't look anywhere near like a fuel but more like "marmelad" xP)

so a better and polished version of that "flamelike" icon from the preview pic would be nice, maybe i try it myself monday when i got some spare time^^
but that blue/white flame in a yellow in a orange flame style is very great already, dispite it's looking ;-)

just my oppinion

I was going for a sort of hybrid between "flame" and "liquid drop," kinda like a "this is a liquid but it hides the potential for inferno" thing which I feel was mostly achieved. I can't draw worth a hill of beans so most of my "art content" revolves around layer and filter tweaking in Gimp (and it shows :p ) or tinting and scaling within Factorio; if anything all I'd change about the current icon is cleaning/sharpening up some of the borders of the droplet and maybe get a nicer color fade from the blue.

Oh also I noticed a bug in my formula in the damage modifier for modded flame turrets so I should probably update. It's a bug that may never occur (modded turret must take light and heavy oil as ammo and have no damage modifier for one of them) so I'm kinda holding off on uploading until I have more changes, though as simple as this mod is I don't know if that will ever happen. So if anybody has any other suggestions...

well, i never got that "spare time monday" - but who ever had spare time on a monday anyway? facepalm

i'm not very "creative" in art related things eighter, whatever (if) comes to my might looks like crap whereever i try to realize it, i'm way better in those things you stated thanks to paint.net and tools written by it's community :-)
(i never got warm with gimp, maybe it's more powerfull than paint.net and maybe more efficient - but hell, it does so much things in such a strange way that i can't adapt to it!)
so i feel with you^^

about suggestions - my experience with flamethrowers so far is that they are already OP as f*** - tough the biggest still burn a while but still can't reach my wall!
(okay, my turrents get the kill...)
so idk about that dmg multipler...

but you might have missed recipes, or am i stupid?
i have the fluid, but i can't do anything with it (not even manufacture it, wtf? o.O)
i looked into chemistry and lvl 3 assemlber, no recipes conected to your mod!
i have no fluid altering mods installed and the item adding mods just work well as expected!
i also use creative menu for testing purposes, thus i can spawn your fluid but nothing else than using it in the turrets...
i restart with only you mod and creative and look back at it...

doublepost
sorry

Kay, it isn't your mod that causes that missing recipes - still no idea wich one as the error appears randomly xP
but i guess it's one of the cheatmods i use for testing purposes.

anyway, i think for the "remaining steel" issue of converting flamethrower ammo into liquid it would be adviseable to leave this option and change it the other way around ;-)
flamethrower liquid can be reconverted into heavy&light oil and then just connected to a chemplant 1:1 to produce flamethrower ammo ;-)
tough that would made it a bit of an exploit as you can transport heavy&light oil via 1 pipe instead of 2 trough your base, but remember at half the rate! ;-)

for the dmg multi after some testing i find it to be not op, as some collossus can run trough it but die rather quickly - but i still need a second line of defence to kill them befor they can hit my walls!
still, i stick with the suggestion to lower it a bit to 115% - and to NOT increase it above 120% as i think it would be OP to defend a base just with flamethrowers!
(as it being the case with the other 2 systems, tough uranium ammo is pretty tough! - not even collossus reached my walls with uranium ammo feeded lines, i experimented with red inserters to increase the gab and still my walls easaly hold them off long enough for my uranium doped turrets to kill them with ease!)