Hessdalen UFOs, color and speed - "anti-gravity", and a link to Tesla:

From Havik's book (the UFO phenomenon), page 177:

"The phenomenon is often seen as reddish-brown, reddish-orange, yellowish-white or blue balls of light. One has often seen that the higher the speed, the whiter the light appear. Stationary balls of light often have a warm-yellow or reddish color."

Enlargement of the 16 mm film that Jon Gisle Børseth took for NRK. This is the "observation UFO", according to Wisth. (Taken from Wisth (1983): "The UFO mystery in Hessdalen")

Hm ... Where have I read something similar?

Well, on this page.

[... In bright sunlight, the aircraft/spacecraft may seem surrounded by hot air, a slight magnetic distortion of the light. In semi-darkness and night, the metal sheets glow, even through the thin ceramic outer hull, with different colors. The visible light is a by-product of the electricity flowing over the metal sheets, according to the frequencies used.

Descending, landing or just starting to lift from the ground, the transformer primaries are near the secondary weak ends and therefore, the bottom set of sheets glow a misty red. Red may also appear at the front of the machine when it is moving forward fast, lessening resistance up front.

Orange appears for slow speed. Orange-yellow are for airplane-type speeds. Green and blue are for higher speeds. With a capacitor addition, making it oversized for the circuit, the blue becomes bright white, like a searchlight, with possible risk of damaging the metal sheets involved.

The highest visible frequency is violet, like Tesla's stage demonstrations, used for the highest speed along with the bright white. The colors are nearly coherent, of a single frequency, like a laser. A machine built with a set of super conducting magnets would simplify and reduce electricity needs from a vehicle's transformer circuits to the point of flying along efficiently and hovering with very little waste of electricity. ]
(Excerpt from "The Lost Journals of Nikola Tesla.")

This represents Tesla's concept of an electrogravitic aircraft, originally conceived in 1919 (according to the source).

I'm no expert on the color distribution of the electromagnetic spectrum, and have even less insight into the potential of electromagnetic propulsion. But the correlation between Tesla's description of his "electrogravitic aircraft" and reports from Hessdalen, I find very interesting.