Why do the color of Jacks eyes seem to change so much?

I was watching the pilot episode again because they were going to replay it and I ran across something that I thought interesting. When Jack first awakens after the crash they zoom in on his face and his eyes. They seem to be greenish-brown. Then as we go along they seem to get darker. I noticed this because in the Chaos Pool story the person that made the decision to give the Doctor the Key of Time in the end and thus saving everything was Zara. She was the person who was impersonating Amy the Doctor’s companion. When the Doctor held a mirror up for her to look into she was revealed to be changing back into her own self.

Excerpt from The Chaos Pool:
http://www.drwhoguide.com/who_bf119.htm

The Doctor reveals to his companion that he knows she’s an impostor and he holds up a mirror that shows her appearance has started to change back into that of Zara. He says he realised the truth some time ago, but he didn’t say anything in the hope that she might lead him to the real Amy.

The Doctor asks Amy if she trusts him and when she says she does, he tells her to let go of the Key. Then he tells Zara she has the choice of either surrendering the Key to the Grace or giving it to him. The Grace threaten to crush the Doctor, but he knows that they’ve manifested themselves inside the Key and they can’t leave it until everyone has let go. Zara asks Amy what she should do, but Amy insists that she makes the choice herself. Zara is angry that the Grace refuse to address her by her name, so she hands the Key to the Doctor, then he turns and jumps straight into the Chaos Pool. There’s a flash and everyone peers over the edge to see that the Pool has turned into a dark chasm full of jagged rocks. Moments later, the Doctor calls up to his friends and asks for their help getting him out.

The Doctor admits that he had no idea there was a ledge just beneath the surface of the Pool. All he knew was that it would disintegrate the Key and disperse the Grace throughout eternity. Everyone who was trapped in the castle segment will be returned home, including Pargrave. He hands the Time Ring back to Zara and invites her to join the man she loves back on Atrios. She’s surprised that he’s willing to let her go after everything she’s done, but he reminds her she was also brave and compassionate. He tells her nobody is perfect and that’s what separates real people like her from artificial lifeforms. The Doctor took a risk giving her the choice, but if he’d compelled her then he’d have been no better than the Grace. He risked everything on his faith that Zara was really a good person. Zara begins to cry. She invites Amy to come with her, but Amy says she has other plans. Zara thanks the Doctor and says goodbye to everyone, then she operates the Time Ring and heads for Atrios.

So with this every time that I saw Jack staring into a mirror since he was the character that did it the most I thought of him as the Zara Character. But I just can’t put my finger on it, his eyes never seem to look the same color especially when for example why are his eyes so light green when he wakes up in the bamboo field in the pilot and when they get back to the island and he ends up in the bamboo field again. When he looks in mirrors his eyes seem way darker. I don’t think this is just my imagination. They say the eyes are the window to the soul. I guess we will see.