'Form Label Colour' is extremely important on MOBLE since you can add forms on any background colour or image, anywhere on your page. Therefore it's important to choose a Label Colour that is clearly legible on top of your backgrounds.
If your website design has light and dark backgrounds, you might consider making two forms variants, one with a light Label Colour and then the other with a dark Label Colour.
In the Forms Area, you can use the Clone button to make a copy of any form, then simply change the Form Name and Label Colour (and possibly the Button Colour), to have both light and dark variants of your Form.