My twin sister OCs for the Collab: Red Shift, and Azure Shift (because both Blue Shift and Blueshift are already taken), they are ponifications of their eponymous astronomical phenomena, "the Doppler effect, but on EMR/light instead of sound".

Azure Shift

The phenomenon of blueshift, is where EMR increases frequency and photon energy, mostly known to be caused by objects moving towards each other.

As such, it's ponification, Azure Shift, is always interested in you, in going towards you, and is very extrovert and energetic!

Red Shift

The phenomenon of redshift, is, well, the opposite of blueshift.

As such, it's ponification, Red Shift, is not as interested in you, she always seeks to go away from you, but don't take this amiss! It's just because she's shy! Oh, and she does lover her twin sister Azure!

The Shift twin sisters (twinsters?)

Just like Red, Azure also loves Red. But they can't touch each other (they phase/are intangible to each other), except for 6 minutes each hour*. They are both pegasi, are very light (pun kinda intended, hehehe), can phase through objects that are invisible to red and blue light respectively, and can move/fly at 0.9c (270 million m/s), but it takes a lot of time and energy for them to accelerate so much, and once they reach that speed, it also takes them a lot of time and energy to deaccelerate.

Their manes and tails represent the EMR change in frequency, in Azure, they are less wavy (lower frequency) on the back, and gradually wavier on the front, and viceversa for Red.

The color gradients represent the color shift:

In Azure: Green to cyan in her mane and tail, cyan to blue in her body to represent the blueshift, and in her eyes, cyan to blue to violet to represent the further blueshifting towards ultra-violet.
In Red: Green to yellow in her mane and tail, yellow to red in her body to represent the redshift, and in her eyes, yellow to red to darker red to represent the further redshifting towards infra-red.

Their intangibility is also represented by their slight transparency, and their wavelike properties are represented by a slight variation in that transparency in the form of a repeating radial gradient.
And the difference in their wavelengths is represented by an even slighter difference in their transparencies.

* This is because they're both the ponification of intangible phenomena, and because it's based on photons, which cannot "touch" each other (because they are free from the the Pauli exclusion principle), but, because having them not able to hug each other seemed cruel to me, I made it so they can touch, and thus hug each other, every time their wavelengths are within a threshold of phase. Something like this: