Lighting: I don't see anything wrong with the lighting you used... I am assuming it was natural light??
Technical: As you've heard before, I think the focus is soft. These are obviously gorgeous flowers, and a sharper focus would have made for a better photo.
Challenge: Can't get more yellow! Definitely met the challenge head on!
Overall: Really a great shot and a wonderful try! I have always struggled with depth of field and focus, both of which would have made this an even better photo. All I can say is practice practice practice. But, I still think it's a great shot and one you can be proud of.
At first wanted to say needs more focus, then thought no, now think yes. It needs better focus in the center, then if you want the outsides a little off, ok. Maybe just sharpening it would accomplish the desired effect. Beautiful flower.