Consider adding package purrple to Remotes:

[lorenzwalthert]

When I try to run the shiny App, I get an error.

collage::shinycollage()
Loading required package: shiny

Listening on http://127.0.0.1:7323
Warning: Error in library: there is no package called ‘purrple’
Stack trace (innermost first):
    42: library
     2: shiny::runApp
     1: collage::shinycollage
Error : there is no package called ‘purrple’
Warning: Error in library: there is no package called ‘purrple’
Stack trace (innermost first):
    42: library
     2: shiny::runApp
     1: collage::shinycollage
Error : there is no package called ‘purrple’

I think you can fix this by adding package purrple to Remotes: (as long as it is not on CRAN), i.e. by adding

Remotes: ThinkR-open/purrple

To the DESCRIPTION file and also add it to Depends: or Suggests: there.