Drop gray from the highlight colors, and let colorFromName return nil

This allows us to fairly easily make gray behave as our legacy grayscale
highlights, honoring the user's opacity value.

ffi/blitbuffer.lua

@@ -2559,7 +2559,7 @@ function BB.gray(level)
 end
 
 --[[
-return a Color value resembling a given hex string
+return a Color value resembling a given hex string (nil on failure)
 --]]
 function BB.colorFromString(value)
     value = value:gsub('#','')
@@ -2601,13 +2601,14 @@ BB.HIGHLIGHT_COLORS = {
     ["cyan"]   = "#00FFEE",
     ["blue"]   = "#0066FF",
     ["purple"] = "#EE00FF",
-    ["gray"]   = "#CCCCCC", -- Matches the default highlight style on grayscale screens (i.e., darkenRect by 0.2)
 }
 --[[
-return a Color value given a common color name (fall back to gray for unknown colors)
+return a Color value given a common color name (nil for unknown colors)
 --]]
 function BB.colorFromName(name)
-    return BB.colorFromString(BB.HIGHLIGHT_COLORS[name:lower()] or "#808080")
+    local color_hash = BB.HIGHLIGHT_COLORS[name:lower()]
+    if not color_hash then return nil end
+    return BB.colorFromString(color_hash)
 end
 
 --[[