I would say that when it comes to creation, there are advantages and disadvantages to each method of art creation. So let us cover these first.
Let's start with standard art. One benefit that standard art has, is that you can usually get away with making black and white pictures that still look good, using only a pencil. I believe that controlling the shade of objects is a whole lot easier than having to pick colors.
Even when you do create color images, it is still usually easier to find colors. I think this is because you generally have a fixed pallet of colors, so you don't have to mess with RGB trying to find the right color (more on that later). If you need to control saturation of a color, all you need to do is control how light you are when applying the color.
Another reason that standard art is easier, is that you have the most intuitive image editor ever! Wherever you put then pencil, is where it draws. It is much easier to control a pencil than a mouse.
Now that I have discussed standard art, I will cover the advantages to digital art and how you can best use your skills for game creation.
One benefit that you get with digital art, is that it is very forgiving. Erasing doesn't leave behind traces of what was once there. This means that you don't have to worry about making a line perfect on the first try, which can make you spend less time on practice draws, and make you more productive.
Another benefit to digital art, is that you have the benefit of layers. This allows you to easily duplicate a part of your image before editing it in case you mess it up. This can also be used to keep parts of an image separate.
But one of the biggest advantages is the fact that your image editor usually provides you with tools that do the work for you. For example, instead of spending a bunch of time looking for reference photos, you can simply click a button, and you have a lens flare, for example.
The final and biggest difference, is that in regular art, digital or not, you are drawing an entire image, containing shading, a subject, and in some cases, scenery. Basically a final image. Where as in game art, you are only drawing individual components. Such as a player (or in some cases, you are only drawing individual body parts), a grass tile, or a weapon.
Based on what I have discussed, I will now discuss how you can adapt. And remember, use the advantages of digital art to your advatage.
First off is color. To make the job of color picking easier, you can create a pallet of colors to use in your image. If you need variation, you can simply adjust the hue and saturation. For this, I recommend switching to HSV.
I also discussed the issue of a mouse being harder to control than a mouse. To alleviate this issue, you may consider using a graphics tablet. A very popular company in the realm of tablets is Wacom. The only problem with Wacom is that they are a bit expensive, which is why I haven't bought my tablet yet. The advantage with graphics tablets, is that they usually have pressure detection. This makes the experience a lot closer to using a pencil. This can be useful for shading.
Philipp's answer (which is very helpful) discussed scale. "You should try to create all 2D assets in the same resolution you want them to have in the final game". And though you want your assets to be the same resolution when exported, doesn't mean you can't create you assets in a different resolution. One trick I learned (I don't remember where I read this) is to start with a large image that can be scaled down to the proper size (I recommend doubling the size a few times. This is also why I like to use image sizes that are powers of two, as 8x8, 16x16, 32x32, 64x64, etc.). Draw the sprite at the large size being mindful that you are going to scale the image down, and then when you are done, scale it down to the proper size. Once you do this, you will usually need to repair edges, and fix small details. Make any additional edits, and save the image at the corrected size. Remember, you may want to keep the large version of the image too.
The final piece of advice I have for switching to game art is making sure you use your editor to your advantage. If you need to make dirt, all your really need to do, is fill a brown area, and then apply noise to the image. And BAM! You have dirt. This is just one example of using your editor to your advantage.
The truth is, that it is hard to understand what you meant by game-ready, but I have tried my best to help answer the question.