So you can say both 'lan se de yan jing' and 'lan yan jing'?
Yes, both are correct.
Why can't you say 她有藍色眼睛？
It's either 她有蓝眼睛 or 她有蓝色的眼睛
It's a mattet of habits. We either say 蓝眼睛 or 蓝色的眼睛
她眼睛是蓝色的 should be right, right? I wrote 她眼睛蓝色的
It will be "Her eyes are blue" instead of the answer