According to Vogue Knitting, the rule is:
If it doesn't specify, slip it purl-wise, UNLESS you are working a decrease, then slip in knit-wise.
It sounds like you're doing mosaic knitting, where you only carry one color at a time, across the row and back, but it gives the effect of two colors in a row. In this case the rule is correct, you want to slip it purl-wise.
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