Yellow is a blankspace in Mathmos' repertoire. Ok, there was the yellow dayglow jet with yellow wax in the early noughties. But as far as I know there was never a yellow Astro or AstroBaby, only the lime green Astros they sold years ago.
Time for DIY!!
I bought two clear-lime Astros and one clear-green Astro, and also some fluoro green and some yellow wax dye. All bottles came very cloudy (and very cheap). I changed the fluid, colored the green wax with the fluoro green dye and one of the lime Astros with yellow dye.
The result is wonderful (Picture comes close, but in reality they all are more saturated and glarier, and the one on the right is indeed a real yellow, not orange).
left to right:
Astro clear-neon green
Astro clear-sun yellow
I own something around 20 Astros, but these are under my top favorites!