The rainbow shape with multi-color effect is possible in text, shapes and images for any Office documents including Word and PowerPoint. With a few changes you can make a rainbow for your front window or other decoration.
Rainbow Text Effect
Insert | WordArt choosing whichever preset is closest to what you want. Select the text and go to Shape Fill | Gradient | More Gradients … then main a rainbow gradient.
Go to Insert | Shape and look carefully for ‘Block Arc’ tucked away in the Basic Shapes.
Resize and position in your document or slide.
Use the two orange dot handles to change the arc shape. The inner (right) handle lets you change the width of the arc to something thinner or much thicker than the default.
For the rainbow effect choose Shape Format | Shape Fill | Gradients | More Gradients.
A text box or slide background can use the background effect too.
Or go rainbow crazy with both text and background as rainbow gradients (different types).