Pretty Rainbows not Murky

This is my first time using the forum … i have a dilema after the weekend cake baking … I made a ‘Frozen’ cake at the weekend and the client wanted a rainbow cake or rather an ombre in pastel blues and lilacs …. I made my usual cake mix as I know its a winner but when I added the blue it went a murky grey green . It only got worse the deeper the shading became on each subsequent layer …. I decided it was because the egg yolks were very yellow that must have been the reason …. either way it was a disaster ….. I am now panicing as I have to make a wedding cake with rainbow layers in a few weeks and I don’t want the same thing to happen…. Can anyone advise me how to get beautiful delicate clean, clear coloured layers please?