I'm looking for various factors; Atmosphere Composition, Type of Sun, etc, whatever would work. That is, of course, if this is even likely/possible. I believe that yellow is generally not an ideal color for foliage, save for attracting bees or something (Which may be myth, I'll have to research that). The environment in question should ideally be able to support human life (not necessarily related) but i can work with it if impossible. No need for paragraphs unless you want to.
The reason plant leaves are green is because they contain the pigment Chlorophyll. Chlorophyll absorbs blue and red light while reflecting green light (thus making it appear green). The absorbed light energy is used for photosynthesis. The obvious way for a planet to have yellow foliage would be for the plants to use a different pigment for their photosynthesis that absorbs all except yellow light. I don't think you need to explain it beyond saying that the plants evolved differently.
That's easy! On Earth, plants use two types of pigments to absorb light for photosynthesis. The most well known of these is chlorophyll, which absorbs blue and red light and thus appears green. But plants also use a class of pigments called carotenoids, which absorb green, blue and violet light and thus appear red, orange, or yellow. It is these pigments that are largely responsible for the color of fall leaves on Earth, as well as the color of carrots, egg yolk, daffodils, and canaries. A class of carotenoids called Xanthophylls are especially yellow compared to other carotenoids which may be more orange.
Carotenoids cannot operate completely on their own, and in Earth plants they must transfer their captured energy to chlorophyll, which then mobilizes electrons into the rest of the photosystem. One can imagine however a setup where the carotenoids transfer their captured solar energy into a molecule that replicates chlorophyll's ability to mobilize electrons but cannot absorb light by itself, or at least is sufficiently shielded by the carotenoids that its color (if any) doesn't show up.
Essentially, you want a photosystem where carotenoids are the primary light-gathering molecules and the separate electron-mobilizing molecules are either incapable of absorbing light themselves (which is more likely evolutionarily, else the carotenoids would not necessary) or tucked away inside dense packets of carotenoids (which makes sense from a structural point of view). As to what these electron-moving molecules could be, I don't know. Chlorophyll does this using a large aromatic ring with a central metal atom, but since these types of compounds strongly absorb light they might not be ideal.
So, what might lead plants to rely heavily on carotenoids and not on chlorophyll? The most obvious solution is to have a planet where the wavelengths of light available at the planet's surface are strongest in the range that carotenoids can absorb but chlorophyll can't. So the available light at the surface of your planet should be:
- Very weak in the red portion of the spectrum.
- Strong in the green portion of the spectrum.
- Very strong in the violet portion of the spectrum.
Violet is your best bet for the strongest wavelengths because a plant that absorbs lots of violet light will appear more yellow. If we focused instead on the green part of the spectrum (where chlorophyll is weakest), the plant would look more red. The increased violet light might make your sky appear more indigo than Earth's pale blue, but atmospheric color is going to depend hugely on what gasses are present and in what concentrations.
Oxygen and nitrogen are pretty good at scattering violet light, but our sun doesn't emit enough of it for our sky to appear more violet. So you can probably achieve this effect simply by making the star emit a little more violet light. I'm treading on the limits of what I know here, but you can probably do this by either by changing it mass to be heavier or just changing its chemical composition (its spectral signature). If more violet light is being scattered by the atmosphere, the sun itself might appear a little more orange than it does on Earth. Regardless, sunlight would still be white light just like it is on Earth, and things on the surface wouldn't be colored differently.
Your planet could be really low on iron.
There is a tree condition called chlorosis that is caused by low iron in the soil, and leads to light green/yellow leaves.
It could also be that the PH in the soil is to high, which keeps trees from absorbing the iron.
Or just go with Tom Andersons idea.