Coconut
Swap some or all of the milk for coconut milk and keep the vanilla light.

A simple iced latte transformed by an electric-blue pour.
Cold drinks
Espresso, cold milk and an electric-blue pour. Keep the layers separate for the drama, or stir it all together and watch the colour change.
The exact amount of blue spirulina isn't precious. Start around 0.25 g for one drink and use a little more or less depending on the blue you want.
The trick isn't more blue spirulina. It's keeping the coffee and blue milk apart long enough for the customer to see them.
Pouring the espresso slowly over plenty of ice gives you that dark coffee layer above the blue milk. As the two meet, you'll get the swirls and gradients that make the drink interesting.
If you mix the espresso directly into the blue milk, expect a much darker, less obviously blue drink. It'll taste perfectly fine. It just won't look like the photograph.
Blue spirulina is easiest to use cold, which makes iced drinks particularly forgiving. If you're having trouble getting it smooth, mix the powder with a small amount of liquid first rather than throwing it straight into a full jug.
And because espresso is so dark, putting the blue in the milk rather than the coffee gives you much more colour to work with.
Swap some or all of the milk for coconut milk and keep the vanilla light.
Skip the espresso altogether for a simple iced vanilla milk.
Use matcha as the contrasting top layer instead of espresso. The blue and green together make a completely different drink.
Make the blue milk ahead rather than measuring a fraction of a gram for every order.
Mix well, refrigerate, and give the bottle a shake before service if necessary.
Once you've found the shade you like, that's your house ratio.

Blue spirulina powder
Choose a pack