Especially when using watercolor paints over an extended period of time, it is possible for the paints to begin to stink. It is totally normal for this to happen. Let me explain why and what you can do to avoid it.
Due to all the natural humidity while mixing paints and painting with watercolors, it is prone to building bacteria. Consequently, the paint smells. The problem can be solved by thoroughly cleaning the paints and palette with hot water. This should be done regularly.

What makes watercolor stink?
As a watercolor artist, you are likely to use the same paints and palette for a long period of time, causing bacteria to grow.
No need to be alarmed or worried; this is a completely natural process. However, you should definitely address this issue, which I will discuss in greater detail later.
We need to identify the root cause of the problem first. Why is the problem occurring in the first place?
The key is humidity, which watercolor palettes, brushes, and paints experience. Since watercolor paintings require water, humidity is inevitable.
The problem is that most artists will not properly care for their materials. An art session does not end when the brush is put away. Your work is done when you clean your supplies.
If not, the humidity might cause mold and bacteria to grow, causing the paint to smell.
Particularly when storing the supplies immediately after painting, there will be no space for them to properly dry.
As a result, the drying process takes a long time, which allows bacteria to grow.
As it turns out, there is a pretty simple fix to all of this, which I will discuss now.
Before, I go on, I’d also recommend reading Are Watercolor Brushes Suitable For Acrylic Paint? once you’re done with this guide. It’s quite an interesting topic.
What can you do to fix stinking watercolor?
The following procedure can be followed if your watercolor pans, palette, or brush are starting to smell weird and unpleasantly:
It is easy to do this and can often be the solution to paints and paint supplies that smell funny.
All you need to do is run all your supplies under hot water and clean your watercolor pans with a cloth or brush.
You can clean your brushes by running them under hot water in the sink. Spread the hair out so that the water can reach all the way. Let it air dry on a flat surface, like a newspaper and it should be clean again.
I recommend cleaning the pans with a medium sized brush. Swish the brush around in the pan after dipping it in clean water, as you would when picking up paint.
Using this method ensures that the pans get clean water, and all the dust and other particles are absorbed into the brush. This only takes 10-20 seconds.
You need to clean this brush properly now, so that all of the paint is gone, so you can use it to clean the next pan.
You need to remove all the paint from the brush so you won’t accidentally mix colors in the next pan.
That should fix the problem with your watercolors smelling weird or bad pretty much.
If it doesn’t go away, repeat the process and add brush soap to it, just keep it out of the pans, so it doesn’t mix with the paint.
By the way, you should also check out my articles about cleaning a watercolor palette, cleaning a watercolor sponge and cleaning watercolor brushes. All of these contain a ton of information and little tips and tricks on the topic. So if you’d like to take proper care of your supplies, I’d recommend checking them out.

What can you do to prevent watercolors from stinking in the future?
Maintaining a certain level of cleanliness is the key to preventing watercolors from developing a strong smell.
You should clean your watercolor supplies (brushes, palette, sponge, paints, cloth) regularly.
Your brushes and sponges should definitely be cleaned after each and every use. Wet brushes and sponges left lying around, which take forever to dry, can create strange odors and breed bacteria.
Fortunately, the process is pretty straightforward. The only thing you need is hot water and maybe brush soap every once in a while. Run the brushes under hot water while spreading their hair, so the water can clean every part of the brushes.
You should deep clean all of your painting supplies every now and then, just to make sure no bacteria is growing.
Brushes and sponges should be cleaned after each use, and the palette every 1-2 weeks, depending on how often you paint.
Once a month, you should do a deep clean.
As long as you follow these instructions, your painting supplies will last a very long time without smelling or accumulating bacteria.
If you’d like to learn more about watercolor painting, I’d recommend checking out What to paint watercolor on! (Lots of options!) and How long does a watercolor painting take?
Summary
Watercolor supplies need to be cleaned regularly. In contrast to brushes and sponges, palettes only need to be cleaned two or three times per month. Each month, you should also do a deep cleaning of all your supplies. This process will prevent bacteria and mold from growing on and in your supplies and prevent odors from forming.