FEATURE: custom colors for default letter avatars (#7167)

lib/validators/color_list_validator.rb

@@ -0,0 +1,14 @@
+class ColorListValidator
+  def initialize(opts = {})
+    @opts = opts
+  end
+
+  def valid_value?(value)
+    hex_regex = /\A\h{6}\z/
+    value.split("|").all? { |c| c =~ hex_regex }
+  end
+
+  def error_message
+    I18n.t('site_settings.errors.invalid_hex_value')
+  end
+end