1. Select the Field to change the label color.

2. Click the 'Advanced' tab and add a CSS Class such as "LabelGreen"

3. Click to edit the Form's CSS

4. Add CSS to specify the color you want to set the <label> inside the field to be.

.LabelGreen label
{
  color: green;
}

In the CSS you can either use Hex code #RRGGBB or a recognized named color. For details on named colored see: https://www.w3schools.com/cssref/css_colors.php

You can use the same CSS class for any fields you want to use the same color or add more CSS Classes for different colors.