Colorado Code
Part 2 - Form, Formation, and Readjustment of Contract
§ 4-2-209. Modification, Rescission, and Waiver

Source: L. 65: p. 1305, § 1. C.R.S. 1963: § 155-2-209.
Prior Uniform Statutory Provision: Subsection (1) -- Compare Section 1, Uniform Written Obligations Act; Subsections (2) to (5) -- none.
Purposes of Changes and New Matter:

However, modifications made thereunder must meet the test of good faith imposed by this Act. The effective use of bad faith to escape performance on the original contract terms is barred, and the extortion of a "modification" without legitimate commercial reason is ineffective as a violation of the duty of good faith. Nor can a mere technical consideration support a modification made in bad faith.
The test of "good faith" between merchants or as against merchants includes "observance of reasonable commercial standards of fair dealing in the trade" (Section 2-103), and may in some situations require an objectively demonstrable reason for seeking a modification. But such matters as a market shift which makes performance come to involve a loss may provide such a reason even though there is no such unforeseen difficulty as would make out a legal excuse from performance under Sections 2-615 and 2-616.
The Statute of Frauds provisions of this Article are expressly applied to modifications by subsection (3). Under those provisions the "delivery and acceptance" test is limited to the goods which have been accepted, that is, to the past. "Modification" for the future cannot therefore be conjured up by oral testimony if the price involved is $500.00 or more since such modification must be shown at least by an authenticated memo. And since a memo is limited in its effect to the quantity of goods set forth in it there is safeguard against oral evidence.
Subsection (2) permits the parties in effect to make their own Statute of Frauds as regards any future modification of the contract by giving effect to a clause in a signed agreement which expressly requires any modification to be by signed writing. But note that if a consumer is to be held to such a clause on a form supplied by a merchant it must be separately signed.
Cross References:
Point 1: Section 4-1-203.
Point 2: Sections 4-1-201, 4-1-203, 4-2-615 and 4-2-616.
Point 3: Sections 4-2-106, 4-2-201 and 4-2-202.
Point 4: Sections 4-2-202 and 4-2-208.
Definitional Cross References:
"Agreement". Section 4-1-201.
"Between merchants". Section 4-2-104.
"Contract". Section 4-1-201.
"Notification". Section 4-1-201.
"Signed". Section 4-1-201.
"Term". Section 4-1-201.
"Writing". Section 4-1-201.