"Bretaña roja" is not a colour; "bretaña" is a type of cloth.

"BRETAÑA: Cierta tela de lino, dicha así porque se trae de Bretaña. Tejido de lino, procedente de esta región, empleado para hacer camisas ordinarias."
Coleccion Pedagogico Textil de la Universidad Complutense de Madrid, p. 9.
http://books.google.es/books?id=DGzLrbBOI9QC&pg=PA9&lpg=PA9&...

I can't find any evidence that this kind of cloth is regularly called "Brittany" in English, and I think it would be best to call it "Breton linen", since that is what it basically means. Breton lace is well known, but Breton linen was also highly prized:

"In the 18th century, around 35,000 people lived off the linen trade in the Côtes-d’Armor region. Around the town of Uzel, many weaving trades were still in operation up to the end of the last century and some examples can be found today at this weaving museum. The golden age of Brittany’s textile industry is a distant memory today, but the museum aims to keep alive this traditional craft by presenting the production stages of the famous ‘Toiles de Bretagne’ linens, known for their extreme high quality."
http://www.brittanytourism.com/things-to-do/all-activities/a...

Here's a reference to "Breton linen (early 1770s)" (pp. 64-65).
http://books.google.es/books?id=lmNF158DvrMC&pg=PA64&lpg=PA6...

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Note added at 5 hrs (2013-04-24 08:16:34 GMT)
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Apparently it was also known historically as Bretagne linen. This text is from 1774:

"All their cloaths are light ; their waistcoat and breeches are of Bretagne linen, and their coat of some other thin stuff."
http://books.google.es/books?id=xr0NAAAAQAAJ&pg=PA521&lpg=PA...