In WordPress Visual Composer, is it possible to set default values for the fields under Select Custom Fields?

Solution:

You can do this by using the “value” property of the parameter array(s) for vc_map() or vc_add_parameter(). Documentation is here… see the description of the value property.

An example would be:

function vc_myshortcode(){
    vc_map({
    'name' => 'My Shortcode',
    'base' => "myshortcode",
    'params' => array(
        array(
            'type'=>'textfield',
            'holder'=>'div',
            'param_name'=>'color',
            'value'=>'#333333'
        )
    )
})}
add_action( 'vc_before_init', 'vc_myshortcode' );

So the default value for the color parameter would then be set to #333333