Hello everyone!

I am a newbee at all of this but I am planning on making a two, 2 tier cakes inspired by Wyland's Underwater Murals.

QUESTION #1:
How do I create an inexpensive way to create an airbrush type effect blue ocean (dark blue from the bottom, lightening up to baby blue on the top)

QUESTION #2:
Can I pre-make the coral and seaweed out of royal icing and then put it on the buttercream cake the day before without it slipping off?

I don't know how to make airbrushed effect without an airbrush, sorry, but yes, you can do the decorations in RI and add them to your cake later, just remember not to refridgerate it with the decorations in place or they will start to melt