In several different early Expanded Universe works set after Return of the Jedi, Luke is shown or said to use a blue-bladed lightsaber:
Luke whipped out his lightsaber, and its brilliant blue glow cut through the shadows. Dozens of aliens screamed and covered their eyes in pain, and many shouted in dismay as they leaped for the door. A dozen rat people scurried deeper into the shadows to watch the fray with glittering eyes.
The Courtship of Princess Leia (1994), page 106-107 of paperback edition; emphasis added
The humming whine of a lightsaber startled him. He whirled around. The blue light filled every corner of the room and even lit a dust-mouse beneath the bed, as if the light were too powerful to cast shadows.
"Where've you been?"
Luke slouched on the deep couch in the corner, wrapped in his robes, his legs extended. The lightsaber flicked off again, plunging the room into darkness.
The Crystal Star (1994), page 114 of paperback edition; emphasis added
In theory, in all of these cases, he should be using the green-bladed lightsaber first seen in Return of the Jedi, and that even seems to be the hilt illustrated for the Heir to the Empire comic. Has it ever been addressed why this choice was made, or if it was just a recurring mistake at the time?
Note: The other arguable examples I found are the cover of The Glove of Darth Vader and interior panels from Dark Empire, but they're not included here since neither one has very strong color fidelity in general.