Offroad diesel can be red or yellow, but you say that you haven't used either, so there are apparently a few other explanations:

What Causes Diesel to Turn Yellow? (4 Common Causes) - Diesel Fuel HQ

Diesel fuel plays a crucial role in powering many engines around the world. It's efficient, energy-dense, and widely available. But if you've ever stored diesel or observed it over time, you might have noticed that its color can change, often turning yellow. Why does this happen? Oxidation: The...

dieselfuelhq.com

Ever question Star Oilco has been asked about off-road or dyed diesel fuel.

An in depth examination of diesel that is dyed red and used for off-road diesels purposes. From heating oil, construction fueling, generator fuel, and agricultural fuel. This article covers every question we have ever been asked. If you don't see your question, let us know we will answer that too.

staroilco.net

 

Where are you buying fuel?
Are you treating it?
If so with what?
Is this a road truck or could someone have tampered with it?

 

I buy my fuel at a cefco. and no one could have tampered with it.

 

Storage tank ?
has water in there -- happy little microbes

 

You should be able to see through a sample that small.
You have a colony of microbes (likely)...
How cold is cold for you?
If that is fuel you got a few months ago, you likely should have treated it with some anti gel of some type. Follow the instructions on the product of your choice, if re-fueling is not in the books you will need to drive and agitate the tank thoroughly. Only if not gelled already. 911 if it is cloudy or gelled.
Also time for this. Or equivalent.

 

@Jimmy's Pilot Service -

Have you used that Clear Diesel before? I have some microbiological (black) growth in some small containers and bought some of that recently to use (haven't used it yet). Would appreciate some feedback.

I noticed that Power Service also sells a biocide:

Bio Kleen Diesel Fuel Biocide Treatment

This dual-phased biocide kills microbes in fuel, including bacteria and fungus. Use this to rid your diesel fuel system of microbial contamination.

powerservice.com

 

I use it twice a year for prevention, I have never had growth in the tank of my truck. If I know the truck will be sitting for a few days I usually time it for a fill up so it can do its thing while siting.
I have treated the tractors on the farm with it in the past for growth and it did kill the microbes. Filters do take a hit. You do need to max out the tank to the cap.
If the tank as sludge it will break it down if you use it regularly so have filters ready.
Clear Diesel is for cleaning the tank and fuel... Nothing else.
Use regular treatment for the other stuff.

 

Appreciated. A friend has an old diesel tractor and I had planned on giving him the fuel (he wants it). I don't think I could bring myself to use it in a 6.0L or even a 7.3L. I will want to clean the containers though, and in future use I wouldn't want to have any increased likelihood of microbes starting up again simply because I didn't kill them completely or didn't clean the containers well enough.

 

I have 15 gallons of yellow fuel like that -- had 40 gallons , but some of it got accidentally spilled on the gravel road in front of my house
Need to look in one of the buckets to see what color it is now

Definitely would not go thru the filters on my dump truck
There is no free water in the fuel , when I drained it
seemed to be emulsified

And when it was "spilled" the consistency was still the same yellow and murky color
Been sitting in covered buckets since last summer , so need to check again to see if it is changed

 

I have 15 gallons of yellow fuel like that -- had 40 gallons , but some of it got accidentally spilled on the gravel road in front of my house
Need to look in one of the buckets to see what color it is now

Definitely would not go thru the filters on my dump truck
There is no free water in the fuel , when I drained it
seemed to be emulsified

And when it was "spilled" the consistency was still the same yellow and murky color
Been sitting in covered buckets since last summer , so need to check again to see if it is changed

Lol, spilled, 25 gallons, sounds like someone had a good time...