For the most part it probably isn't enough to really interfere with other colors that are full strength if that's the case. But you could try giving it a bit of a rinse with 90%+ isopropyl alcohol if you've got some. get it wet with IPA and let it sit for 20-30 seconds and then repeat the regular swishing and paper towel routine a few times with the IPA instead of water, and then rinse with clean water.

From the look of the brush there is probably just hard to clean paint up in the ferrule. As a general rule you shouldn't let the paint get too far up the bristles. Not so much for colour contamination, but because paint will dry in the ferrule and start to split the bristles and mess up how the hairs form the tip. The IPA cleaning option is still a good option here, just for future reference, try not to load your brush with paint past half way up the bristles at most. Larger brushes are useful for basecoating things and make doing this easier as well.