Borane fuels (including pentaborane and diborane, among others) burn with a green flame
Nicknamed the "Green Dragon" fuel for the characteristic green exhaust produced by the combustion process. These fuels have one of the highest theoretical ISPs of any fuel combinations along with Dioxygen difluoride (FOOF, both its name and what it does when you even think about it wrong). However, it is majorly toxic, completely pyrophoric (explodes on contact with air) even when not totally pure, corrosive, reacts with water and many halogenated fire extinguishing compounds, and is considered dangerous in concentrations above 1 part per million in air. The US destroyed its stockpile of 1900 pounds of pentaborane in 2000 after a way to decompose it safely was conceived.