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Jack, the preamble tells us to highlight the thematic area (91 cells). I don't see the need to use two colours, unless you are treating the thematic location as separate from the group members.

I too have sometimes used a different colour when the rubric says “highlight”. I tend to do Listeners in pencil, so inking over with a colour is easier sometimes than shading a background colour which risks drowning out my pencil! I hope it’s allowed as a reasonable interpretation: I’d hope that anything that indicates the intended area would be accepted, and take a little more care when the instruction is more specific (eg “shade in red”).

I was travelling when the penny dropped and had no highlighters with me, but did have a couple of biros in different colours. I am sure it is entirely acceptable to use that method.