Tamanho e cor do ícone mdi
<div class="input-group">
<input type="text" class="form-control" placeholder="Search...">
<span class="input-group-btn">
<button class="btn btn-default" type="button" aria-label="Search">
<i class="mdi mdi-magnify" aria-hidden="true"></i>
</button>
</span>
</div>
Holy