That is not always the case.
A buddy has a Buell, it is marked incorrectly....
He was having all sorts of brake problems after changing fluids (if I remember correctly it had to do with Brembo rubber parts) .
Did some research and found out it was marked wrong.
Flushed and replaced all the rubber parts of the system and the brakes work great now.

 

This is so on point to what the OP is trying to find out that it should be quoted.

OP this ^ is the answer to your question. It'll take 2 minutes to figure out and it'll take 2 hours to rebuild the master cylinder and caliper(s) if you introduce silicone fluid into a glycol system or vice versa. As far as flushing lines and going from one to the other? Not me, without a complete rebuild of everything including lines. A frozen caliper because of the eventual goo isn't on my list of things I'd like to do.

 

When i bought my bike used, it only had 8000 miles on it. But when they were adding some chrome pieces to it, they added a chrome front master cylinder cover and bought the wrong one as far as which fluid it used. I wouldn't imagine that someone had the wrong stock one on the op's back, but stranger things have happened.

Head

 

I flipped an 04RKC a couple years ago and it came with dot 5. I disagree where someone said dot 5 will turn a straw color, thats dot 4. Dot 4 looks more like a vegetable oil or has a yellowish tint. Dot 5 is a purpleish color when new, and turns black when dirty. Ive done a conversion before on a bike but I used an engraver and made a 4 over the 5 after replacing everything when I did the conversion. Person didnt want to buy a new cap.
Sounds like your rear is original and the front has been compromised. Youre between a rock and a hard place, knowing it is wrong and now you have to do it right.

 

even my 16 says DOT 4 on the caps, and they make a synthetic DOT 3/4 as well.

 

That's because of your ABS. Harley switched from DOT 5 silicone to DOT 4 glycol in 07? when they brought out ABS.

The OP's 04 RK should have come stock with DOT 5 silicone brake fluid, afaik.

 

All Dot 3/4 brake fluids are synthetic. The important difference we're talking about here is glycol based and silicone based. A completely different thing.

 

My bike is an '04 Road King. I just looked at an old receipt from the last brake job performed at the dealer, (10 yrs. ago). Part # 99953-A Dot 4 brake fluid. That's all I've ever used.

 

It'll work. That said, take a look at your service manual or your owners manual. You'll see DOT 5 is what is specified. Part number 99902-77.

 

The "purplish" color doesn't last for long...not a good way to determine what is in your bike.