Add memoized versions of rgb_colors for foreground and background c…

…olors.

The changes were made to improve performance by reducing the number of
calculations required to find the nearest color for a given RGB value.
The `rgb_foreground_colors` and `rgb_background_colors` methods only
return colors that match the specified options, which can help reduce
the amount of data that needs to be processed.

lib/term/ansicolor/attribute.rb

@@ -49,12 +49,25 @@ def self.get(name)
         @__store__[name.to_sym]
       end
 
-      def self.rgb_colors(options = {}, &block)
-        colors = attributes.select(&:rgb_color?)
-        if options.key?(:gray) && !options[:gray]
-          colors = colors.reject(&:gray?)
+      class << self
+        memoize method:
+        def rgb_colors(options = {}, &block)
+          colors = attributes.select(&:rgb_color?)
+          if options.key?(:gray) && !options[:gray]
+            colors = colors.reject(&:gray?)
+          end
+          colors.each(&block)
+        end
+
+        memoize method:
+        def rgb_foreground_colors(options = {}, &block)
+          rgb_colors(options).reject(&:background?).each(&block)
+        end
+
+        memoize method:
+        def rgb_background_colors(options = {}, &block)
+          rgb_colors(options).select(&:background?).each(&block)
         end
-        colors.each(&block)
       end
 
       def self.named_attributes(&block)
@@ -63,14 +76,12 @@ def self.named_attributes(&block)
 
       def self.nearest_rgb_color(color, options = {})
         rgb = RGBTriple[color]
-        colors = rgb_colors(options)
-        colors.reject(&:background?).min_by { |c| c.distance_to(rgb, options) }
+        rgb_foreground_colors(options).min_by { |c| c.distance_to(rgb, options) }
       end
 
       def self.nearest_rgb_on_color(color, options = {})
         rgb = RGBTriple[color]
-        colors = rgb_colors(options)
-        colors.select(&:background?).min_by { |c| c.distance_to(rgb, options) }
+        rgb_background_colors(options).min_by { |c| c.distance_to(rgb, options) }
       end
 
       def self.true_color(color, options = {})

lib/term/ansicolor/attribute/color256.rb

@@ -40,7 +40,7 @@ class Color256
         Attribute.set :on_color4, 4, html: '#000080', background: true
         Attribute.set :on_color5, 5, html: '#800080', background: true
         Attribute.set :on_color6, 6, html: '#008080', background: true
-        Attribute.set :on_color7, 7, html: '#c0c0c0'
+        Attribute.set :on_color7, 7, html: '#c0c0c0', background: true
 
         Attribute.set :on_color8, 8, html: '#808080', background: true
         Attribute.set :on_color9, 9, html: '#ff0000', background: true