Imported Library Colors

What’s the significance between imported libraries displaying in orange text vs black?
For example:
#include <CountingStream.h>
#include <Xively.h>
#include <XivelyClient.h>
#include <XivelyDatastream.h>
#include <XivelyFeed.h>
#include <b64.h>
#include <HttpClient.h>
#include <SPI.h>
#include <Ethernet.h>
#include <Dns.h>
#include <SD.h>
#include <OneWire.h>
#include <stdlib.h>
#include <Time.h>
#include <EthernetUdp.h>

Some libraries have words in their keywords.txt file, eg.

#######################################
# Syntax Coloring Map For OneWire
#######################################

#######################################
# Datatypes (KEYWORD1)
#######################################

OneWire KEYWORD1

...

That means OneWire will be coloured orange. However it has no other significance. Not every library has a keywords.txt file, nor do they necessarily colour every word.

I see. Thanks