The author has referred to a pool of water in the river; and to the colour of water - the colour of tea.
He combines these two ideas to convey an image he has, involving the girls sitting on the bank of the river. They are wearing light-coloured dresses (dresses with pale colours) and made of cotton material; and because they are sitting, the material in the skirt of the dresses (the part of the dress below the waist) would be on the ground, extending out and around each of them, as if they were each sitting in a pool of something: and the image is, sitting in 'pools of pale cotton'.
Get it? The pale cotton material of the dress is like a pool around them, and they are sitting in it.
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