Update introduction.md

Correct default value

docs/usage/introduction.md

@@ -128,7 +128,7 @@ You can alter the default paper by using the following function:
 
 ### Default font
 
-By default, every text appears in Arial 10 point in the color red (FF0000). You can alter the
+By default, every text appears in Arial 10 point in the color black (000000). You can alter the
 default font by using the following two functions:
 
 ``` php