feat: add color string format rules

AGENTS.md

@@ -103,6 +103,10 @@ Use the helper style already present in the package you are editing.
 
 For rules, keep table data close to the rule test and cover both passing and
 failing inputs.
+Organize each rule's test declarations as one adjacent group in this order:
+test-case inputs, the `Test*` function, and the `Benchmark*` function.
+Finish that group before declaring the next rule's test cases.
+Place shared test-case types above the rule groups and shared helpers below them.
 When expected output includes validation messages, assert the exact message
 unless the existing package uses a looser helper for that case.
 

cspell.json

@@ -36,6 +36,8 @@
     "Elts",
     "Fset",
     "Messagef",
+    "RRGGBB",
+    "RRGGBBAA",
     "Skłodowska",
     "atoi",
     "cidrv",
@@ -70,4 +72,4 @@
     "Implicits",
     "mygovy"
   ]
-}
+}

internal/messagetemplates/templates.go

@@ -54,6 +54,10 @@ const (
 	StringJSONTemplate
 	StringSemverTemplate
 	StringCVETemplate
+	StringHexColorTemplate
+	StringRGBTemplate
+	StringHSLTemplate
+	StringDeviceCMYKTemplate
 	StringContainsTemplate
 	StringExcludesTemplate
 	StringStartsWithTemplate
@@ -128,8 +132,15 @@ var rawMessageTemplates = map[templateKey]string{
 	StringJSONTemplate:        "string must be a valid JSON",
 	StringSemverTemplate:      "string must be a valid semantic version",
 	StringCVETemplate:         "string must be a valid CVE ID",
-	StringContainsTemplate:    `string must contain the following substrings: {{ joinSlice .ComparisonValue "'" }}`,
-	StringExcludesTemplate:    `string must not contain any of the following substrings: {{ joinSlice .ComparisonValue "'" }}`,
+	StringHexColorTemplate:    "string must be a valid CSS hexadecimal color",
+	StringRGBTemplate: "string must be a valid{{ if .Custom.LegacySyntaxOnly }} legacy comma-separated{{ end }} " +
+		"CSS rgb(...) or rgba(...) color",
+	StringHSLTemplate: "string must be a valid{{ if .Custom.LegacySyntaxOnly }} legacy comma-separated{{ end }} " +
+		"CSS hsl(...) or hsla(...) color",
+	StringDeviceCMYKTemplate: "string must be a valid{{ if .Custom.LegacySyntaxOnly }} legacy comma-separated{{ end }} " +
+		"CSS device-cmyk(...) color",
+	StringContainsTemplate: `string must contain the following substrings: {{ joinSlice .ComparisonValue "'" }}`,
+	StringExcludesTemplate: `string must not contain any of the following substrings: {{ joinSlice .ComparisonValue "'" }}`,
 	StringStartsWithTemplate: `
 {{- if eq (len .ComparisonValue) 1 -}}
 	string must start with '{{ index .ComparisonValue 0 }}' prefix

pkg/rules/error_codes.go

@@ -34,6 +34,10 @@ const (
 	ErrorCodeStringJSON                    govy.ErrorCode = "string_json"
 	ErrorCodeStringSemver                  govy.ErrorCode = "string_semver"
 	ErrorCodeStringCVE                     govy.ErrorCode = "string_cve"
+	ErrorCodeStringHexColor                govy.ErrorCode = "string_hex_color"
+	ErrorCodeStringRGB                     govy.ErrorCode = "string_rgb"
+	ErrorCodeStringHSL                     govy.ErrorCode = "string_hsl"
+	ErrorCodeStringDeviceCMYK              govy.ErrorCode = "string_device_cmyk"
 	ErrorCodeStringContains                govy.ErrorCode = "string_contains"
 	ErrorCodeStringExcludes                govy.ErrorCode = "string_excludes"
 	ErrorCodeStringStartsWith              govy.ErrorCode = "string_starts_with"