Your requirements are so specific that I would say: don't search for some existing product which has it all, just engineer your own.

The simplest would be to take some base which is transparent and does not have a strong color in itself and whose sweetness can be varied. For example, make your own elderflower syrup with whatever proportion of elderflower and sugar you like for the taste. Use food coloring for the color, and pectin to get the desired jellylike consistency.

Don't can the jam, as the arbitrary amount of sugar won't allow it to be shelf stable. Just keep it in the refrigerator and it should be fine until it grows moldy (how long it takes will depend on the sugar concentration). Freezing is also an option and "pauses the clock" on expiration.

Okay, so I think the obvious answer is dandelion jelly. Yes, that is a thing that exists. You could probably safely add some other edible, yellow flowers (like chrysanthemums, maybe? or even some oolong tea?) and almost certainly cut back on the sugar, but that's a little more risky and will require some experimentation. I have added gelatin to my jellies before in order to cut back on the sugar without losing structure, but that will change the texture a bit.

I also googled "bright yellow juice" and found a detoxing "super yellow juice" that looks interesting without being gross. There's a picture near the top that is quite a bright yellow. That said, I don't know how the color would hold up to cooking.

This is their recipe (since you have to scroll really far to find it):

2 yellow bell peppers (stem and seeds removed)

1 rutabega (peeled)

2 organic golden delicious apples (cored and seeds removed)

1 Pear (optional -- it's sweeter with it, a little more tart without it...)

2 Lemons (peeled)

2 inches of fresh ginger (skin removed)

As far as jelly-making goes, if you start with a juice, you'll need to strain it thoroughly in order to get a clear-ish jelly. That will require cheesecloth and coffee filters and lots of time. For the cooking part, you need pectin and sugar to form bonds to get the proper structure. This means that you will need to add some sugar to your juice, and that once you have your juice/sugar mixture, you'll need to cook it until it reaches about 220F. That's when the pectin and sugar should bond up properly. Worst case scenario, you get some thick, gloppy syrup instead of what you think of as "jelly."

Lemons and apples are both good sources of pectin, so if you include those, you should have no worries about pectin itself. The rest is chemistry involving water, sugar, and acidity, which I am not an expert on, but the fruit itself should provide the necessary acidity, and cooking to 220F should give you the correct amount of water. As for the sugar-- what I know is that I have cut the sugar in jelly recipes in half and had no issues with structure. This has mostly been the case with cranberry sauce, which is super high in pectin, but I usually leave out at least a quarter of the sugar in jelly recipes with no issue-- sometimes I do get a gloppy mess. The gloppy mess still tastes good, though, so I don't worry about it.

But as I mentioned above, you can introduce structure with things like gelatin, agar agar, or other less familiar thickeners that don't require sugar to set up properly. I wouldn't rely entirely on non-pectin thickeners, for sure, since they tend to have an unpleasant taste in large amounts, but in reasonably small amounts it can help, and any taste will be covered up by the existing sugars in the fruits you use.

Apple jelly with either food coloring or turmeric in it. Not sure if you can just mix stuff into a jar of already made jelly, and keep its nice original texture, but you can make the jelly from apple juice (many recipes available on internet, e.g.: http://www.cooks.com/recipe/x16tm5jm/apple-jelly-from-apple-juice.html)

Turmeric has such a strong color, that I think if you used an amount small enough so that the resulting taste was "interesting", ... but still nice, it would still give you your "vibrant yellow".

Of course before doing a lot of work, I would experiment with store-bought apple jelly first.

Sounds to me like a pineapple jelly would do the trick for you. It stays vibrant yellow, and a jelly instead of a jam would be clear with no floating pieces or chunks.

Pineapple is a fairly unusual jelly/jam flavor, but quite good. It makes an interesting but delicious pb&j, and could probably be substituted pretty much anywhere you would use jelly or jam.

Unfortunately it is likely to be quite sweet, but you could likely adjust the ratio of juice to sugar to find one that suits you (the two below have wildly different ratios, so I think you could definitely play around with it-- its just a question of the minimum amount of sugar necessary for the jelly to set properly).

Example recipes: http://www.kraftrecipes.com/recipes/certo-pineapple-jelly-51940.aspx https://www.justapinch.com/recipes/sauce-spread/jam/pineapple-jelly.html

I've seen a yellow garlic-lemon sauce... it was very good, and I put it on many sandwiches, though the color was not quite what you want. I don't have a recipe (it was commercially made) but perhaps you can develop something of the sort, tweaked for the appearance you want.

The garlic lemon sauce I mentioned was opaque and pale, possibly mayonnaise based. A butter or oil based sauce would probably give a better color - for example, the commercial garlic butter sauce (used as a dip) can be much more yellow, and sometimes a bit less opaque.

The texture is still pretty soft, though it might be able to be thickened with a corn starch slurry. I have had a corn-starch thickened soup so thick it was almost gloppy (and really amazing, texture and flavor worked very well), close to syrupy but not quite at a jelly-like consistency. It was also a really beautiful vibrant gold color - it was a broth based soup, and might have had some coloring - or turmeric - to help give that lovely deep color.

So, between the two, you might have something pretty close. A butter-based sauce, which will thicken and firm up when cool, might be mixed with a gloppy corn-starch based sauce. And the results might well reach syrupy or even jelly-like consistency. The flavor pairing of garlic and lemon is already well-received as a sauce, and may be well complemented by the savory flavors in a golden broth - there are vegetarian golden broth packets available, such as G. Washington's brand, or you can make your own vegetable broth and even deepen the color with turmeric. Butter will, I think, give a better flavor, but oil will tend towards translucent... you could try a mix, depending on how it all tastes.

Butter and lemon (especially zest) will give paler yellows, the broth will give golden yellows, and if you add a bit of turmeric (very bright yellow) I think you should get a pretty vibrant yellow color out of it.