adobe photoshop - How to achieve a wet and dark outline like this

I'm using adobe illustrator and I am putting watercolor effect on my elements. I want it to look like this:

I have added a watercolor background to the element, but then my art looks very plain and flat:

I want to add a thick outline to the art to make it look like the original. Or here are some more examples:

Answer

None are perfect, but I can think of a few options...

You can add an inner glow effect (Effect → Stylize → Inner Glow...) to your shapes, just set your glow to a darker color and a more appropriate blend mode...

This is a dark grey filled shape, set to "Multiply" at 80% opacity over a texture image, with a black inner glow also set to multiply:

The same shape with a gradient fill, a colored inner glow and the texture clipped to that shape:

Another option is to use a custom brush stroke:

Another option is to use the Appearance panel...

Add two fills, the bottom one set to the color of your "outline", the top one to your desired fill, then add two effects to the top fill; an "Offset Path" with a negative value to bring it in, this will give the appearance of a stroke; then a "Roughen" effect to add the rough details.

You could take it further by adding a slight blur or whatever else, the "Offset Path" and "Roughen" is a good start though: