[API] Fix inconsistent label color format (#10129)

* update and use labelColorPattern

* add TestCases

* fix lint

* # optional for templates

* fix typo

* some more

* fix lint of **master**

modules/structs/issue_label.go

@@ -22,7 +22,7 @@ type CreateLabelOption struct {
 	Name string `json:"name" binding:"Required"`
 	// required:true
 	// example: #00aabb
-	Color       string `json:"color" binding:"Required;Size(7)"`
+	Color       string `json:"color" binding:"Required"`
 	Description string `json:"description"`
 }