While most of the card games described in Charles Cotton's Compleat Gamester of 1674 are well attested both before and after its date of publication, the game of Costly Colours is a notable exception. The Oxford English Dictionary mentions it only as "an obsolete game at cards" and offers, besides a second-hand reference to Cotton, just one citation:
They found Duroy and Heartley playing at Costly Colours: a game upon the cards peculiar to that country.
from W. Toldervy, The History of Two Orphans (1756). (Country means county, and presumably Shropshire is meant, as will become apparent.) Cotton's account is supplemented by some aberrant additional comments from Randle Holme (1688).
I should probably not have paid much attention to so ill-served a game were it not for two happy coincidences. One of these was that Arthur Taylor, pub-game researcher and author, once happened to mention in conversation that he had seen a peculiar sort of three-card Cribbage played in a Lancashire pub in the early 1980s, and that he thought it was called something like "Costly". Unfortunately he was not aware of its significance, and by the time he went back to find it, there had been a change of management and no one seemed to know anything about it. The other had previously come my way in 1975 by courtesy of Robert Reid, then of the Queen's University of Belfast, who kindly sent me a photocopy of some pages describing the game taken from Charlotte Sophia Burne's Shropshire Folklore, 1883. Here is the introduction to it:
The following digest of the game of Costly, now  obsolescent, was made partly from oral instructions given by "old players", partly from rules set forth in a scarce hand-book kindly lent for the purpose, entitled "The Royal Game of Costly Colours." "Printed for and sold by J. and W. Eddowes in the Market Place, Shrewsbury, 1805." In the Advertisement the Editor says, "The way in which the game was first introduced to his notice was as follows:- Having a few years ago taken up his residence in a village in Shropshire, whenever be was invited to spend an evening with his neighbours, rarely any other Game at Cards was talked of but the Game of Costly Colours, and which therefore he was told he must learn. Several considerations weighed with him," he says, "to be at the pains to collect all the Information respecting it that the oldest players in the village, assisted by some of his acquaintance at Shrewsbury, were competent to give him,... for the purpose of introducing it amongst his Friends elsewhere: and seeing his expectations not disappointed in the pleasure he promised himself it would afford them, he is induced to present it to the public, and particularly to the admirers of the Game of Cribbage, with the like confidence that they will admit its advantages in the comparison with that game."
In 2014 Clive Dean, of Shrewsbury, drew my attention to a card-playing scenein Mary Webb's novel Precious Bane (1924), which in its time was considered so risqué that some libraries refused to stock it. It's about life in the north of Shropshire in 1815 and the chapter describes Costly Colours being played by women at a betrothal party. I've scanned the relevant passage from pages 98 and 102 of the Penguin edition:
"You'll need more gumption than you've got then," says Missis Sexton, "for there's no game so hard as the game of Costly Colours. I've played it at every randy since I was a maid, and I'll lay that your Ma has too, and Missis Sam and Missis Miller. Yet it's a difficult game to us still. And for you that have played it seldom or never, it'll go hard, but you'll lose every cake." Tell 'em the way of it," says Missis Beguildy, "you've got such a head." [...]
She went across the kitchen like a coach and six, and stood by the fire, telling us about the game of Costly Colours - how you counted, and of the trumps, and how three of a suit was a prial, and four of a suit was Costly, and how you could mog, or change, your cards, and of the deuces and Jacks, and "Two for his heels," and how if you made nought of your hand it was called a cock's nest, and you were bound to give a cake all round. [...]
"Two for his nob!", called Missis Sexton. "Your deal, Prue."
Points (in play or hand)A note about sequences As in Cribbage, a sequence is three or more cards in numerical order, for which purpose Ace is always low (A-2-3 is a sequnece; Q-K-A is not). The cards of a sequence need not be played in sequential order, but the sequence must not be interrupted by any duplicated or extraneous card.
15 scores 1 per constituent card
25 scores 1 per constituent card
31 scores 1 per constituent card
Jacks & Deuces (in play or hand)
Jack or Deuce turned up pegs 4 to the dealer ("for his heels", if a Jack)
Jack or Deuce of the turned suit in hand pegs 4 to its holder
("for his nob", if it is a Jack) (Note 2).
Any other Jack or Deuce in hand pegs 2 to its holder
Pairs and prials (in play or hand)
Pair (two alike) 2
Prial (three alike) 9
Double prial (four alike) 18
Sequences (in play only) Peg:
1 hole per card in sequence.
Colours (in hand only)
Three in colour 2
Three in suit 3
Four in colour, two in suit 4
Four in colour, three in suit 5
Four in suit 6 for "Costly Colours"
1. Thus says "Salop", anonymous author of the piece in Shropshire Folklore, but not mentioned by Cotton. (Return 1)
2. "His nob", according to Salop, denotes the Jack or Deuce of a suit other than trump. I query this, and here follow Cribbage terminology. (Return 2)
3. Or so I assume; neither source specifies. (Return 3)