Nasturtiums (noun) – any of a genus (Tropaeolum of the family Tropaeolaceae, the nasturtium family) of herbs of Central and South America with showy spurred flowers and pungent edible seeds and leaves.

Source – https://www.merriam-webster.com/dictionary/nasturtium

In “Quicksand”, “Only a single reading lamp, dimmed by a great black and red shade, made by pool of light on the blue Chinese carpet, on the bright covers of the books which she had taken down from their long shelves, on the white pages of the opened one selected, on the shining brass bowl crowded with many-colored nasturtiums beside her on the low table, and on the oriental silk which covered the stool at her slim feet.” (Larsen, pg 35)

I now understand that the author is trying to set a seen with the description of the nasturtiums, which are colorful flowers that can be red, yellow, and white. These flowers are next to Helga Crane as she sits around a table.