When you do a strand test, check it carefully in every possible light. Don't fool yourself - if you think you see the slightest green or blue, or even just 'ash' tinge, than that's what a head of highlights will produce. If you only did the one indigo, you'll probably find that you can go ahead, but be prepared to have to then cover them with what should probably be a permanent, in a gold/warm blonde or something like that.
I'll be doing a few highlights again soon, even over my traumatised blue-green (but hidden, by then, under gold dyes and henna). Even so, I will be expecting the re-emergence of some blue-green, and will have a bottle of something ready to cover them!
My first stage took place tonight, which was the application of a medium warm brown PPD thingy to my roots. A little lighter than the darkest parts on the lengths, but that's fine with me, as I'm looking to achieve a warm mid-dark red-brown effect in the final analysis.
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