Directly relating to the Morrigan we admittedly have only a few pieces of color related evidence, but we do have some.
From the Tain Bo Regamna:
"A red-haired woman with red eyebrows was in the chariot with a red cloak around her shoulders"
"... he saw that she was a black bird on a branch near him."
"I will be a blue-grey wolf-bitch then against you," she said.
"I will be a red-eared white heifer then," said she...
From the Tain Bo Cuiligne:
"...a smooth, black eel"
"...a rough, grey-red bitch"
"...a white, red-eared heifer"
From the Cath Mag Rath:
"She is the grey-haired Morrigu"
Additionally we see Badb referred to repeatedly as 'red-mouthed' or 'the Red Badb', for example here in the Cath Maige Tuired Cunga: "The Red Badb will thank them for the battle-combats I look on.". In the Tochmarc Ferbe Badb is described as a 'white woman' or 'shining woman' and in the Destruction of De Choca's Hostel she is also said to be red-mouthed and pale. Black would be associated with her through ravens and crows.
Macha, has less blatant references to color so more guesswork is required. As Macha Mongruadh [Macha of the red-mane] she would seem to be associated with the color red, something we may also with less surety say due to her being called 'the sun of womanhood' in the Rennes Dindshenchas. Her association with skull could perhaps give us the color white for her, although that in itself is an assumption based on her explicit connection to severed heads and the wider Celtic cultural use of skulls. Black is easier as she is clearly connected to crows and ravens, and grey is also a color connected to her through the hooded crow and through the most famous horse known to be hers [before he was known to be Cu Chulainn's] the Liath Macha, literally 'Macha's Grey'.
All three of the Morrigans [Morrigan, Badb, and Macha] are said to take the form of hooded crows, birds which are black and light grey, and of ravens or crows more generally. In several stories including the Tain Bo Cuiligne the Morrigan is said to appear 'in the form of a bird' and one may perhaps assume the bird here was meant to be understood as a hooded crow or raven. In the Sanas Cormaic they are called the 'three Morrigans' and later 'raven women'. In one version of the Aided Conculaind we are told "And then came the battle goddess Morrigu and her sisters in the form of scald-crows and sat on his shoulder". The names Badb and Macha are also words in Irish that mean crows or hooded crows, reinforcing the connection between the Morrigan(s) and the color black as well as grey.
|By Zeynel Cebeci (Own work) [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons|
Black had connotations of dark, dire, melancholy, and was used to express intensity, something like the word 'very' in english.
White represented purity, brightness, holiness, truth but also bloodlessness and was sometimes used to describe corpses. It was also a color in combination with red that was often used to describe Otherworldly animals.
Red* was used to describe things that were bloody, passionate, fiery, fierce, proud, guilty (think red cheeks) also used as an intensive.
Grey usually represents age, in the plural the word for the color means 'veterans'
So we can see that when color is mentioned in association with the Morrigan it is usually red or black, and slightly less often white or grey, and rarely blueish-green. I might suggest that people who associate red, black, and white with her are either consciously or subconsciously picking up on these patterns from her stories, particularly of the colors of her animal forms when contesting with Cu Chulain in the Tain Bo Cuilinge which are black (eel), red (wolf), and white (cow), although the red/black/white pattern is not limited to that. Badb and Macha share these color associations in different ways, indicating that it is not the Morrigan as a singular being for which these colors are important but rather that all three Morrigans relate to them.
*there are actually multiple words for the color red in Old Irish; I am using 'derg' here which is the one most often used in the texts to describe the Morrigans, et al, however it is not the only red used so that should be kept in mind.