Hello and welcome to the forum 
Welcome to digital painting and welcome to krita too.
For krita specifically, it’s a good idea to read or consult the manual which is here: Welcome to the Krita 5.0 Manual! — Krita Manual 5.0.0 documentation in case you hadn’t already found it.
Which drawing tablet have you ordered?
Also, which operating system are you using?
For tablets in general, please read this section of the manual for background information:
In case of problems, please read this section:
If you learn on your own with experimentation then you’ll certainly have some frustration and miss out on lots of things but you’ll also stumble on some interesting tools and techniques. Watching and reading tutorials is a very good idea of course.
Many people here will be able to recommend tutorials for general and specific use and I’m sure you’ll get lots of suggestions soon.
In the meantime, you can check out the suggestions and links in this topic: Where to learn digital painting (with krita) ?!
The image you posted has the mushroom body and stalk as transparent, which is why it’s showing as black on the website page. You may have made a simple mistake or have a basic misunderstanding of how to deal with layers and exporting images.
Can you take a fullscreen screenshot of the krita window while the image is open, with the Layers docker visible, then post it here to get some guidance and advice?
For total beginners in digital painting, it’s always the image layers and the saving and the exporting (for posting and viewing) that causes problems and confusion. After that, there are other things that will cause confusion but they can be dealt with later
A ‘cheap and easy’ way to get blending is to use the Blender Blur brush preset but you’re probably right that you should have used more hues. You can also maybe reduce the opacity slider for your brush while painting to get a layering and build-up effect.
I’m sure other people can tell you better ways of getting good blending.