Are you using liquid or gel food colouring? I’m not familiar with the blue velvet cake and what shade of blue it should be but I’ve made a few rainbow cakes and have been able to achieve good colour using gel food colouring and Rose’s white velvet butter cake, an all egg white cake (and even the all-occasion yellow downy cake, an all egg yolk cake). I’ve included a photo of a slice. I use the Americolor brand and find that the cake colours are very true to the colour of the gel, though I’m sure other brands would work.
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