document.getElementsByName("field_name")[1];
you have to specify [1] because the fields are placed in a div with the same name so [0] is the div and [1] the field itself.
Hope that i helped !
Léovar field_div = document.getElementsByName("name")[0];
var label = field_div.getElementsByClassName("sys_f_label")[0];
// some logic here to decide color
label.style.color = "red";
This code gets the div that contains all of the field's components, and then retrieves the element inside that div with the class the client uses for styling labels.
Eli