If the parents are not sure what the sex of their baby is and they don’t like the idea of just sticking to the old yellow color angle, then you could suggest a rainbow baby shower. This way, you can include other ideas to make the decorations lovelier like clouds, rainbows and birds, even rainbow bears if you really like them.
Of course looking for invitations with rainbows on them are aplenty so you could actually use these to write in the details of the party. Send them out to friends with a note at the back as to the attire and where to find the gift ideas.
Because the colors are ROYGBIV, then of course you can use all the colors of the rainbow for your decorations, including the shape of the rainbow itself. Instead of a painted arch, why not make an arch of the rainbow using different colored balloons? You could also make a tent in a room using multi colored swaths of cloth tied to the center of the ceiling and draped across to the walls.
You could also have flowers of every kind and color for the centerpieces. Place then on tables covered in cream or white to accentuate but not detract the eyes from the varied colors of the rainbow. Set the table with white plates, but the table napkins can be differently colored, like the rainbow.
On the buffet table, place an arch overhead of a balloon rainbow, and a centerpiece of varied flowers of all colors. It could be one main centerpiece or it could be three or four medium sized pieces place together.
For the food, you could serve it according to the acronym of ROYGBIV also: Red for spaghetti, Orange Gateau, Green salad, Blueberry cheesecake, Beef Stew, Indigo colored drinks and Violet tinted cakes. The rest of the food can be Orange chicken, Grilled pork and other delicious goodies. Arrange the food according to the acronym too. It can be confusing, but it is a change from the usual.
The cake can be the main focus again on the center table. It could actually be multicolored just like the rainbow. Cut this in time for dessert and serve to the guests with hot coffee or hot tea.
Serve drinks that would suit the food like sodas, juices, light beer, iced teas and carbonated fruit flavored water. If serving alcoholic drinks, make sure to assign some designated drivers whether or not you need one.
Offer up the party favors at the end of the party. Wrap some goodies in tulle tied with ribbons and a Thank You Card. Inside place some lollipops, pastel covered candied almonds, gummies, and probably a lip balm and glycerin soap.