### Steps to reproduce

1. Be an administrator

2. Go to Admin -> Colors

1. Either create a new color OR

2. modify an existing one

3. Provide a Begin of the insertionan erroneousEnd of the insertion code Begin of the deletion without the '#' signEnd of the deletion in the "Hex code" Begin of the insertioninput field:

1. use less than 6 hexadecimal characters like "#", "#AB", "#ABCD", or "#ABCDE" (note that "#ABC" is ok and is expanded to "#AABBCC")

2. add the leading "#" sign multiple times like "##ABCD" or "###ABC"

3. use non hexadecimal characters without the leading "#" like "ll" or "hello"


End of the insertion Begin of the deletion input 


End of the deletion ### What is the buggy behavior?

* 500


### What is the expected behavior?

1. Potentially, in the absence of a '#' sign, just assume it to be there and be robust about it.


### **Logs**

```text
app/models/colors/hex_color.rb:111 String#%
app/models/colors/hex_color.rb:111 Colors::HexColor#blend
app/models/colors/hex_color.rb:97 Colors::HexColor#darken
app/helpers/colors_helper.rb:130 ColorsHelper#icon_for_color
app/views/colors/_form.html.erb:46
app/views/colors/edit.html.erb:37
app/helpers/application_helper.rb:338 ApplicationHelper#labelled_tabular_form_for
app/views/colors/edit.html.erb:34
app/controllers/colors_controller.rb:86 ColorsController#update
```

https://appsignal.com/openproject-gmbh/sites/681379ae55b0b20ed3735994/exceptions/incidents/1078