Tab bar item icons appear darker

I can see that you already got an answer on stackoverflow, so just let me sum it up here quickly.

The Apple Documentation says the following on UITabBarItem:

By default, the actual selected image is automatically created from the alpha values in the source image. To prevent system coloring, provide images with UIImageRenderingModeAlwaysOriginal.

That is, colors provided in the images are ignored by default when set on an UITabBarItem, but rather system defaults are applied. To ensure images are displayed like you intended them to be, provide a standard and selected icon as in the following example:

...
myTab.image = [[UIImage imageNamed:@"iconStandard.png"] imageWithRenderingMode:UIImageRenderingModeAlwaysOriginal ];
myTab.selectedImage = [[UIImage imageNamed:@"iconSelected.png"] imageWithRenderingMode:UIImageRenderingModeAlwaysOriginal];
...

Also worth a read: The iOS Human Interface Guidelines on tab bar icons.