Angular - How to apply [ngStyle] conditions

[ngStyle]="{'opacity': is_mail_sent ? '0.5' : '1' }"

For a single style attribute, you can use the following syntax:

<div [style.background-color]="style1 ? 'red' : (style2 ? 'blue' : null)">

I assumed that the background color should not be set if neither style1 nor style2 is true.

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Since the question title mentions ngStyle, here is the equivalent syntax with that directive:

<div [ngStyle]="{'background-color': style1 ? 'red' : (style2 ? 'blue' : null) }">

You can use an inline if inside your ngStyle:

[ngStyle]="styleOne?{'background-color': 'red'} : {'background-color': 'blue'}"

A better way in my opinion is to store your background color inside a variable and then set the background-color as the variable value:

[style.background-color]="myColorVaraible"