Grace wasn’t sure about the contrast between the 2 different yarns. Here’s a picture of 2 bitty scraps, one from each yarn.
Pretty darned close, no?
There are color overlaps between the two yarns that make them nearly indistinguishable from each other. The second yarn is mostly black based, but the color sections are similar in hue and value of the darker sections of the rainbow yarn. I think it will work. If it doesn’t? There’s always a dye bath.