When you do a retouch on weaved hi-lites how do you make sure you dont go back over a allready beached area?

I understand you hit the new growth at the roots, but sometimes there will be a strand mixed in thats colored. do I try to pick those out? or do I only bleach the root area, will that look ok?

Most times the colored strand was blone before it was colored, due to haveing color and re-touch of hi-lites before. Do you understand what I am asking? sorry if its confuseing.