Coloured shading

I have had a go at shading a rainbow using the same colours as on the building blocks.

Unlike in the blocks, here yellow is a problem. Red/orange and green/blue/purple work fine but the transitions in and out of yellow are very clear, especially the one between it and green. I'm not sure if the problem is the move from a warm to a cold colour or if that particular shade of green is too dark. I am going to have another go using different thread colours to see how that compares.