For caption text with invisible fill, the font color has to be either a 4 character code that ends in zero or an eight character code that ends in double zero.
You have to uncheck the Font Shadow box but specify an outline value.
Here are the color values I used (one of them is partially hidden in the image)
"font_color": "#fff0",
"outline_color": "#22ee22"
"font_color": "#ffffff00",
"outline_color": "#ff3333"