Question: I was reading this [article](http://www.riagenic.com/archives/526) about Metro UI designs, and I was struck by this statement:

> 
> Example, if you look under the hood of a new car, you will see areas
>  colored yellow whereas the rest is the same color of the engine etc.
>  Car manufacturers do this on purpose, they want you to touch yellow
>  yourself but if it is not yellow and you are not mechanically minded –
>  leave alone.
> 
> 
> 

The author recommends using a color like Yellow as the **secondary color** as a means to draw people’s attention to the fact "I’m ok if you touch this, you won’t break anything if you do" thinking .I am a little unsure about this mentality that if you see something which is marked a different color other than the primary or background color,you would immediately mark it as a safe.

What are your views on it?

Answer: 
I think the example from the post makes reference to a key concept: grouping by color (probably described by a [Gestalt Principle](http://www.vanseodesign.com/web-design/gestalt-principles-of-perception/), but I can't find a reference to it now).

**In a site that has a prominent hue, an alternative hue will appear to be highlighted.**

Take as an example [this A/B testing](http://blog.hubspot.com/blog/tabid/6307/bid/20566/The-Button-Color-A-B-Test-Red-Beats-Green.aspx).

![red wins green](https://i.stack.imgur.com/0GiPY.png)

Both green and red are bright colors, but because the general hue of the site is green, the red button —which would normally convey caution— had better conversions.

The same happens with the car example: the motor is something that you don't need to touch. Anything that has te same color as the engine is *grouped* —psychologically— together. Whatever stands out, be it in yellow or any other color, will be likely to get the user's attention.
That example illustrates some cognitive psychology, named here as grouping. Works splendid!

More examples of visual cues: 
<http://www.eruptingmind.com/depth-perception-cues-other-forms-of-perception/>

Perhaps you had something else in mind? If so, please elaborate your question.

Edit: Gestallt principles that is how it's called:
<http://www.interaction-design.org/encyclopedia/gestalt_principles_of_form_perception.html>