Caption colors can be set in the Meme Generator.
Imgflip updated the site yesterday (2019-05-17) how user specified colors are stored. Now 3,4,6 & 8 character colors codes work.
https://en.m.wikipedia.org/wiki/Web_colors
Basically what that means is transparency is also supported now for all the usual colors when you use 8 characters where the usual 6 character color code would go before. Right now you have manually enter an 8 character color code, that might change if the Imgflip color picker is updated with a second slider or something like that.
The last two characters of an 8 character color code specifies 256 levels of transparency, 00 is fully transparent, ff is fully opaque.
The trick to both translucent and transparent captions is turning font shadow off, also using font outline is pretty much the only way to go with transparent captions.
Shout-out to user VagabondSouffle who showed me how to make translucent captions a couple days ago with 4 character color codes.
The caption settings I used are shown in my comment image.
The color codes for the translucent and transparent captions in my meme.
Top caption:
"font_color": "#eeee0077",
"outline_color": "#000000"
Bottom caption:
"font_color": "#ff00",
"outline_color": "#ffffff"