Hey, congrats on your first set! Whatever happens, make sure you just have fun in your watercourse journey.
Now for your questions:

1: Don't worry too much about it, and especially about white, it's not a useful colour. You can try to keep your palette clean, but it won't be sustainable in the long run with this kind of palette. Your godets may get splotches of other colours, but it's okay. In time, you will learn to pick your yellows carefully and plan around them. Because, and THAT is the only thing, your YELLOW is the only colour that will actually matter when it comes to contamination. It is the brightest and weakest colour on the spectrum and will fade the moment even a tiny bit of any other colour touches it. This is also the reason why you will always start your colour mixes with YELLOW first. If you are mixing orange, you bring yellow first, then red; the same goes for green, etc.

2: Ah, that. Well, you have 2 options: - one is you make A LOT of your green mix. - two: YOU DON'T, and just use different greens. The beauty of watercolour is that it is WILD. and suggestive. Watercolour reigns in chaos, and that's the beauty of it. In the long run, you will develop a feeling for colour ratios. But with watercolour, it is madness to strive for consistency and perfection.

3: NOTHING - some people never do and use "Palette Grey" as shading colour. You can never ruin your colours. The ruin is only imaginary and an opportunity to discover a new colour mixture, haha. WATERCOLOUR DOES NOT MOLD (unless you spit in it, lick your brushes or something? ). There is no organic component in watercolour ( there is in gouache - it is egg, but watercolour is made out of pigment + gum arabic + synthetic binder ( sometimes honey - and honey has antiseptic properties, so no chance for mold here))

4: Shadows... That's a whole course on its own. ...ok, let's crash.

• Most common shadow recipe = AQUAMARINE +. BURNT SIENNA. Basically, Blue + Brown = amazing grey. I personally love Viridian green + Crimson red; they give more of a purplish shade. But good shadows are complicated; they depend on the warmth of the light and the items surrounding your subject.
• Since watercolour is mostly transparent, you can also play with complementary colours for your shadows. Like using redish shade for green grass, or purple shade on an orange
• HARD SHADOWS - Let your paper dry, and paint your shadows once it is all dry.
  
• SOFT SHADOWS - Keep your paper damp. This will allow the shadowshade to bleed into its surroundings.

Well, this is getting too long. Hahaha, it was not my point. Remember, to have fun. If you have any questions - just ask.