Fix handling of basic PB items in Item Getting Guide

Okay so, like 30 minutes ago I added fallback behavior for cases where
we can't correctly infer the color from a PB item's name… and then I
pulled it up in the color and found that, oh, right, there are already
3 PB items that *correctly* return `nil` for `Item#pb_color`: Aisha
Collar, Elephante Hat, and Ixi Collar.

This is because they're common items that apply to many colors, like
the basics, but also many other less-special or older color variants.
They are the most likely case where we'll return `nil`.

So, I've updated our fallback UI to, instead of talk vaguely about
missing data, just assume that we're dealing with basic items. In the
rare window of time where a new color is released, and we have PB items
for it but no manual color data yet, this can just incorrectly say
"Basic Colors" and that's fine.

app/views/items/sources.html.haml

@@ -71,11 +71,14 @@
 						= image_tag pet_type_image_url(@pb_color_pet_types[color], size: :face),
 							srcset: ["#{pet_type_image_url(@pb_color_pet_types[color], size: :face_2x)} 2x"],
 							alt: @pb_color_pet_types[color].human_name
+					- else
+						= image_tag "https://images.neopets.com/items/starter_red_pb.gif",
+							alt: "Item thumbnail for Starter Red Paint Brush"
 				%th
 					- if color
 						#{color.pb_item_name || color.name.humanize}
 					- else
-						??? New color
+						Basic colors
 					(#{pluralize items.size, "item"})
 				%td.actions-cell
 					- if color&.pb_item_name?
@@ -90,7 +93,7 @@
 							Get via Lab Ray, morphing potions, etc.
 					- else
 						.special-color-explanation
-							We don't have info about this color yet.
+							Many colors, like Red, will grant this item.
 			%tbody
 				- items.each do |item|
 					= render "item_list_row", item: