Use a single tab field separator in keywords.txt

Each field of keywords.txt is separated by a single true tab. When you use multiple tabs it causes the field to be interpreted as empty. On Arduino IDE 1.6.5 and newer an empty KEYWORD_TOKENTYPE causes the default editor.function.style coloration to be used (as with KEYWORD2, KEYWORD3, LITERAL2). On Arduino IDE 1.6.4 and older it causes the keyword to not be recognized for any special coloration.

Reference:
https://github.com/arduino/Arduino/wiki/Arduino-IDE-1.5:-Library-specification#keywords

keywords.txt

@@ -1,7 +1,7 @@
-SIMM		KEYWORD1
-begin		KEYWORD2
-read		KEYWORD2
-write		KEYWORD2
+SIMM	KEYWORD1
+begin	KEYWORD2
+read	KEYWORD2
+write	KEYWORD2
 readBlock	KEYWORD2
 writeBlock	KEYWORD2
-stop		KEYWORD2
+stop	KEYWORD2