Ethanol/oxygen is my guess. Blue, and also very little soot.
It's hard to say for sure, but I lean towards Honda's rocket using hydrogen. Honda has experience with it. They use hydrogen in their fuel cell vehicles, and their press release from 2021 mentions using hydrogen for rockets.[1]
I'm pretty sure both fuel and oxidizer are cryogenic, because when the rocket lands it vents from several areas (most likely separate tanks). That would rule out ethanol or methanol as the fuel.
I don't see any secondary exhaust from a gas generator, and staged combustion would be something to brag about (and much higher thrust), so my guess is that it's an expander cycle. Expander cycle engines require a fuel that boils easily, so it would have to be fueled by propane, methane, or hydrogen. I don't think it's propane, as the only propane/lox rocket I've seen has orange exhaust.[2] If Honda poached some engineers from Mitsubishi, I could see them going with a hydrolox expander cycle, as that's what the H family of rockets use.
The only thing that doesn't line up with hydrogen is the low thrust given the propellant consumption. Based on the claimed wet/dry mass (1,312kg/900kg), they used at most 412kg of propellant. Flight duration was 56.6 seconds, so that's an average of 7.28kg of propellant per second. If the stated wet/dry mass is correct and the rocket used up all of its fuel, then the rocket's thrust was around 13kN at the start and around 7kN near the end. Let's say it averaged 10kN. Force equals mass flow rate times exhaust velocity. So 10kN divided by 7.28kg/sec is 1.374 km/s. Divide by standard Earth gravity and you get 140 seconds, which is pathetic for a rocket. It could be that they only used a small fraction of the available propellant, or they had a poor nozzle design, or the engine was throttled very low and was therefore less efficient. If we assume the test flight only used 40% of the available propellant, then we'd get a flow rate of 2.9kg/sec and a specific impulse of 352 seconds. But that sort of assumption can be used to come up with any Isp.
Still, I think it's using either hydrogen or methane as fuel. Nothing else fits with the video.
1. https://global.honda/en/newsroom/news/2021/c210930beng.html
2. Here's a video of Isar Aerospace's Spectrum rocket, which uses propane/lox: https://x.com/isaraerospace/status/1906418985173758236
It could simply be pressure-fed. No turbopump at all—just a helium tank.
You have a preference for assuming sophistication, but this is a one-off inexpensive test article with trivial performance needs. My guess is that they'd made the simplest engineering choices possible at every turn.
Solid boosters put out the most visible exhaust, as burning APCP[1] emits solid particles of metal oxides. Also some rockets (mostly Russian, Chinese, and Indian) use unsymmetrical dimethylhydrazine + dinitrogen tetroxide, which emits a reddish-orange exhaust. Both compounds are toxic, as is the exhaust.
1. https://en.wikipedia.org/wiki/Ammonium_perchlorate_composite...