Using a flower outline as an example, I will show you how to create a smooth outline that is well-integrated with the color of the object. Our goal is to hide the outline with a smudge technique to make sure that the outline of the object is made with colors, not with a plain line.
For this tutorial, I have used the software "Painter essentials 4" from Corel. I have used the soft grainy round brush with water colors to create the main lines and the background colors. The digital airbrush was then used to complete the lighting and shadows effect, but this tutorial can easily be done using real tools like chalks, pencil or paint.
First, sketch the top of the flower using a yellow tone for the center and a light gray tone for the petals. Like you can see on the example above, you don't need to sketch a perfect flower. The goal is only to learn how to create a nice outline!
Then, draw the stem and the leaves using a light green tone.
Next, draw an outline on each shape using a tone that is a little darker than the shape's color you are working on. Once again, notice that most lines are not perfect at all.
Now try to smudge the center a little bit and add some lighting effects using a tone of yellow that is darker than the center, but lighter than the one used for the outline. Our goal is to make an outline that is subtle and filled with a gradient effect.
Apply the same technique for the petals. Try to add a darker tone on the bottom of each petal since we are presuming that the light is coming from above.
Finally, do the same for the stem and the leaves. However, this portion might be a little harder to do since the shape is narrower. Never mind if it's not perfect. You will have the chance to make correction on all imperfections in the next (and final) step.
It's now time to finalize your artwork. Apply some white color on all areas that are exposed to lighting. Don't worry if you don't have the desired effect right away! It's pretty normal to work with light and dark tone simultaneously in order to obtain the right effect!
That's it! Now you should have a pretty good idea on how you must work with various colors in order to make a flower outline that is smooth and very subtle! I hope you had fun and happy drawing!