The top portion of the cake thats green has this shine...what is it? Pearl dust?? 

Looks like it's airbrushed with pearl sheen.

ATry steam - a very fine mist

Can anyone tell me the main difference between pearl dust and luster dust?  It seems like luster dust gives more metallic finish which I want.  But how can I get a similar look without a airbrush?  Also when purchasing it, I dont see many choices in color.  I want it use over blue  fondant, would the silver one be ok?

Brush it with pearl luster dust then. 

AThe two products are similar, both give you a sheen. You can get green luster dust, maybe use that.

Is there something that's sort of like transparent yet shiny?  I'll be working on a bubble guppies cake...so the background will have blue fondant and green as the seaweed....I would like to add something that will make everything still show their colors but be shiny. 

Pearl shimmer, pearl luster, there's a few different kinds. Make it easy on yourself, get Wilton pearl (whatever it's specifically called). It's in a little clear tube. Yes, your color will show underneath. Just don't cake it on.

Thanks for all the advice, I found Wiltons silver mist and it was perfect.  At first I did a small spray but realised I wanted the entire cake and the characters to look like it's under water so I added a few more layers of the spray and my niece loved it. 

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