Potential sources for this information include both written and visual accounts of the clergy. For the latter, we can look to illustrations in manuscripts, bearing in mind that the technical constraints of these illustrations mean that drawings may well be simplified in terms of colours and patterns. Also, artists may draw clergy as they "ought to be" rather than as they really were - if you need to show that someone is a bishop, you draw them in stereotypical bishop clothes. It's also important to look for sources of the correct time and place, as lonesomeday observes in comments above.
Some examples of bishops from manuscripts in the British Library include:
Another interesting data point is a 13th century stained glass window in Canterbury Cathedral, showing Thomas Becket. Again, the colours will partly be due to the technical possibilities and artistic demands of stained glass, but they probably can't be completely wrong for the popular image of a bishop.
As far as written sources, there is a tension between the rules for what clergy should have been wearing, and what they actually wore in practice. There is a long history of canon law trying to enforce modest dress, which in itself suggests that these laws were frequently being stretched or ignored.
Canon 16 of the Fourth Lateran Council (1216) says that clerics "are not to use red or green garments or curiously sewed together gloves, or beak-shaped shoes or gilded bridles, saddles, pectoral ornaments (for horses), spurs, or anything else indicative of superfluity." We can certainly deduce that some clergy before this date were accustomed to wear such garments. And even afterwards, there are wonderful texts like the 1342 canons of John Stratford, Archbishop of Canterbury, saying:
[The clergy] scorn the tonsure ... and distinguish themselves with hair hanging down to their shoulders in an effeminate manner: and apparel themselves like soldiers rather than clerks, with an upper jump remarkably short and wide, with long hanging sleeves not covering the elbows: their hairs curled and powdered, and caps with tippets of a wonderful length, with long beards, and rings on their fingers, girt with girdles exceeding large and costly, having purses hanging with knives like swords in open view; their shoes chequered with red and green, exceeding long and variously pinked; with croppers to their saddles, and baubles like horns hanging down on the necks [of their horses], and cloaks furred on the edges.1
England is unusual in that clerical dress was less regulated than in mainland Europe. The canon law commentary by William Lyndwood (1429) asserts that English clergy have no fixed rule of dress, but can wear anything they like "appropriate to their estate", so long as it is not explicitly prohibited.2 His argument is that this is the long-established custom in England, which therefore prevails over ordinary legislation. This is later than your date of 1215 but the customs he refers to are older. Lyndwood's commentary says that the restriction on red and green implicitly also forbids clergy from wearing striped or multicoloured clothing - though, again, some of them undoubtedly did, even if they were not meant to.
In English canon law, the constitutions of Walter Gray for the province of York (1250) and of John Peckham for Canterbury (c. 1280) do not establish any particular colours for liturgical vestments, or for non-liturgical clerical use; though they do have norms for which garments should be worn, and who is responsible for them. The only colour references I see in Johnson are the Lateran prohibition of red and green, and the rule in Walter's Canons of 1200,
Let not black monks or canons or nuns use coloured copes, but black only; nor any facings but black or white, made of the skins of lambs, cats, or foxes.
Given the date, this cannot refer to the Dominicans (conventionally "black friars") but must be the Benedictines ("black monks"). For abbots, practice varied probably as much as it did for bishops, although the sources of law were different. In theory, they should have been wearing basically normal monastic clothing, appropriate to their order. However:
- Many monks, contrary to law, wore other things anyway.
- An abbot was very likely to also be a priest, and some of them were consecrated as bishops.
- Many English abbots had the privilege of dressing as if they were bishops, even if they were not consecrated as such ("mitred abbots").
Therefore, your twenty abbots might have been difficult to distinguish from the bishops and archbishops. Also, as far as I know, there's no colour that distinguishes an archbishop from a bishop.
The papal legate, Pandulf, was not a cardinal, and in 1215 he was only a subdeacon. However, I think that since he was a high-ranking legate (legatus a latere) he would have worn the red hat. This was the standard mark of direct papal representatives, which was later extended to all members of the college of cardinals, and which lent its colour to their other clothing. So he could well have been dressed as a subdeacon but with the wide-brimmed red hat we now think of as a cardinal's hat; or he might have worn more elaborate clothing, given the stature of his diplomatic mission.
1. From John Johnson's Collection of the Laws and Canons of the Church of England, vol. 2, 1851.
2. Tamen non habent certum Habitum eis deputatum nec in colore nec in forma; & ideo
possunt qualicunque veste usi, eorum statui congruenti, dummodo eis non sit expresse prohibita. Source.