Szerintem a problema abban van ahogy a ColorConverOp-ot letrehozod/hasznalod.
Javadoc:
http://docs.oracle.com/javase/8/docs/api/java/awt/image/ColorConvertOp…-
"This Op can be used only with BufferedImages, and is primarily useful when the filter method is invoked with a destination argument of null."
Nalad a filter target nem null.