You could try ditching the built-in field constructors and instead write your own. The following template accepts a custom argument that controls the styling of the label:
app/views/_my_field_constructor.scala.html
@(element: helper.FieldElements)
<div class="clearfix @if(element.hasErrors){error}">
<label for="@element.id" class="@element.args.get('_label_class)">@element.label</label>
<div class="input">
@element.input
</div>
</div>
Now use your new field constructor instead of whichever built-in one you were using before:
app/views/form.scala.html
....
@* implicitFieldConstructor = @{ FieldConstructor(twitterBootstrapInput.f) } *@
@implicitField = @{ FieldConstructor(_my_field_constructor.f) }
....
When calling the helper function to create a input text field, you can now pass in a custom _label_class
argument that the template will pick up:
app/views/form.scala.html
@inputText(orderItem("item1"), '_label -> "Product", '_label_class -> "red", '_class -> "tinytfss")