You can use most of those verbs with いる in Japanese. In English, the verbs like to have is considered as stative, right? Cuz the motion to initiate the action (in this case, have) ends for one moment and you go enter the state of having something. In Japanese, almost all those verbs are still considered as motion verbs that has the start and end. So when you say 持つ, it refers to the action to initiate the state (in English, grab etc.). Thus, when you enter the state of the action, it has to be 持っている. Hope it helps!
「ろくに」has some negative connotation.
It’s often used when you want to talk down on something/somebody. So in this case, you may just want to say あまり持っていません.
I couldn’t find a way of saying “I haven’t learned ‘grey’ yet”. I managed “I didn’t learn grey”, but wasn’t sure how to translate the English use of haven’t which conveys that something will happen eventually.
Please correct any grammatical mistakes/oddities - I’m still getting that!