Its hard to say without seeing the image and your results or even what steps you took.
Most likely this is because of Anti -Aliasing This is where pixels on the edge of two colours are a Mix of those two colours. This makes it difficult to get a clean edge when making a selection. Again with out seeing what you have done suggestions are guesses. But if you selected the bull and filed it with red and it looks good Contract the selection ( Selections. modify. contract) by 1 pixel to get rid of the edge then promote selection to a layer. You should now have the red bull on a layer of its own and ca either copy and paste as a new image and save for further use, Or just flood file the original background layer.
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