What are requirement of anicon for Tab Bar item ?
From Tab Bar Item class reference:
The item’s image: If nil, an image is not displayed. The images displayed on the tab bar are derived from this image. If this image is too large to fit on the tab bar, it is scaled to fit. The size of an tab bar image is typically 30 x 30 points. The alpha values in the source image are used to create the unselected and selected images—opaque values are ignored.
You can't use color images as only alpha values matter.