The rainbows I think is the key instruction given in the beginning of the book. The rainbow has 7 colors that can be put in order, this can be handy once we know what rainbows are. It could reffer to color code, RGB code, the colors simply can be words pointers where we have to use words or letters around it to spell out something. The rainbow also can have a metaphorical meaning, as finding something that stands out, and possibly stringed together as phrase (a rainbow) among the ton of other words in the book (reffered as rain).
For what it's worth, I haven't been able to find any occurrence of the word 'orange' in the book. Every other color from the primary rainbow is present. Red, xxxxxx, Yellow, Blue, Green, Purple
It is odd that orange doesn’t appear, we can assume the apricot marmalade is orange but the word orange itself doesn’t appear. And looking at what colors are technically part of a rainbow, indigo is included, no mention of that either.