Hola, Xavier,
jo normalment faig servir aquests quatre tipus de blocs:
<div class="alert alert-success" role="alert">
This is a success alert—check it out!
</div>
<div class="alert alert-danger" role="alert">
This is a danger alert—check it out!
</div>
<div class="alert alert-warning" role="alert">
This is a warning alert—check it out!
</div>
<div class="alert alert-info" role="alert">
This is a info alert—check it out!
</div>
Cadascun es mostra d'un color diferent.
Com deia el Joan, l'has de posar fent clic a la iconeta d'HTML i et quedarà així:
This is a success alert—check it out!
This is a danger alert—check it out!
This is a warning alert—check it out!
This is a info alert—check it out!
Fins ara,
Mònica