Tanning injections aren't exactly uncommon. They can't be legally administered in a salon setting as botox or filler can because they haven't been tested and approved as "safe" by regulatory boards. Thus, they're a bit more "underground" for the general public, in a similar way to anabolic steroids which your typical gym addicted, Ibiza dwelling duck boy would be well acquainted with.
Pure speculation of course, Danny likely tans very well, uses sunbeds and "may have" tried tanning injections in the past
☀⛱