25,5:The return of the Demons

The Return of the Demons is a special extra-long episode that comes between the 24th and 26th of Dragon Ball After, Towa has summoned Evil Buu and sent him to the Earth to kill all the '''Z-fighters. Goku Jr.''' sends everyone to King Kai's planet because he knows that they will never win... So he sacrifices himself by facing him alone. Goku Jr. had learned very much from Goku and had spent 2 days in the hyperbolic chamber (which means he accumulated 2 years of training) But he was definitely not strong enough to face the completely new Buu.

Semi Super Saiyan
When Goku Jr. fought against the revived Kid Buu, he knew

he was not strong enough, he needed to transform.

Until then Goku Jr. had only achieved the first Super Saiyan transformation, he had no experience nor knowledge about the Super Saiyan transformations, he only knew they had more ascended transformations than just one. So he tried to collect al the Ki he could find and tried to transform into the long haired Super Saiyan he saw in his dreams (Super Saiyan 3).

Goku Jr's hair did have the form of a Super Saiyan but was green colored (a bit darker than Legendary Super Saiyan hair). Also his aura was not visible anymore, instead, he had red colored electricity 'zapping' around him and sometimes his hands created some ball shaped aura (image). His clothes also got light colored.

The reason this all hapenned was because Goku Jr. did not have this much Saiyan roots anymore (since the last full Saiyan his family was Grandpa Goku himself). He can now be called a Half Blood Saiyan, this makes creatures with smaller Saiyan roots (starting at 1/16 Saiyan) need less effort to transform into a normal Super Saiyan but it's almost impossible for them to ascend the Super Saiyan form.