You can add a subview to the parent tabBar, and set a background color on the subview. You can use the tabBar frame dimensions to calculate the offset and width of your tabBarItem, and then insert the subview underneath.
Example (in Swift):
// Add background color to middle tabBarItem
let itemIndex = 2
let bgColor = UIColor(red: 0.08, green: 0.726, blue: 0.702, alpha: 1.0)
let itemWidth = tabBar.frame.width / CGFloat(tabBar.items!.count)
let bgView = UIView(frame: CGRectMake(itemWidth * itemIndex, 0, itemWidth, tabBar.frame.height))
bgView.backgroundColor = bgColor
tabBar.insertSubview(bgView, atIndex: 0)
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…