Why does it work the same with square brackets and without brackets?

I have a question:

<p appAttributeDirectives [color]  =  green >

Look at the attribute color is in a directive that inside has an input that would be the alias and the green attribute is the component.Ts I do not understand why I take the brackets

<p appAttributeDirectives color  =  green >

Same works

and also if I add quotes to the gree attribute the same works

<p appAttributeDirectives color  =  "green" >

in that three cases works but I do not understand with what purpose those brackets and quotation marks would go if in any case the same would work if those symbols so:

<p appAttributeDirectives [color]  =  green >