In this 20-Nov-2018 Tweet from Rocket Labs XXX Peter Beck there is an image of the nine Rutherford engines of an Electron rocket burning. What is unusual is that the exhaust is luminescent but at the same time transparent.
Stage one is tested and ready to fly for our December ELaNa 19 mission for NASA! Electron is headed to the pad next week for checkouts.
Usually RP-1/LOX rocket exhaust at least appears to be opaque due to optical absorption of all the carbon-based soot and gas, although over-exposure may contribute to that visual effect as well.
I'm wondering if the image in that tweet (below) is just underexposed, or if a special filter has been used, or perhaps it's not using visible wavelengths and has been artificially colorized.
A quick analysis shows that the yellow color is not just a yellow tint added to a monochrome image, there is some variation within the image. The drop in overall intensity with decreasing wavelength (r g b) suggests the yellow could be the actual color of the exhaust.
above: Cropped, zoomed, and sharpened.
below: Similar, found in Facebook during image search: