Question: We've been designing a registration form with 5 steps.

The form has many input fields, some required and some not.

The validation is client side and is represented with red text for errors.

I was wondering if success messages along with error messages would provide positive feedback and in some way influence the user to keep motivated.

Maybe there are other ways to do that, and maybe too many green icons in front of form fields can become quite ugly.

Answer: 
The user must feel that they are in control of the form and they should be aware how long the form is. 

If it's a long form you can have titles at the top or the side marked as "sections" or "steps" and change the color (or change the font color)once they have been completed, like the example below that shows that the user is in screen 3.

But also you can give the user instant validations with html5 to assure that as they go they can see the mistakes and finish the form quicker (rather than clicking on "submit" and then getting the error or even worse, lose all the data they have typed because the page refreshed itself).

![validation](https://i.stack.imgur.com/W9ztL.png)
Don't give trivial feedback, as it would take attention unnecessarily. 
However, there are fields that a successful input in not trivial.

One good example would be the username. When you type your new username, there's a chance it's taken. When a proper one is entered, you can indicate that the chosen username is valid.

![First example](https://i.stack.imgur.com/uTT0Q.png)
![Second example from twitter](https://i.stack.imgur.com/LlX1Q.png)

And another nice example (though a bit excessive):

![enter image description here](https://i.stack.imgur.com/MSNnV.png)

In an afterthought, after looking at the last example, the positive feedback might provide some encouragement to complete the form.