Problem with defining a background color by variable

[suzee]

Hello,

I want to change the background color of code (and other) blocks in 
org-mode as proposed here:

https://orgmode.org/worg/org-contrib/babel/examples/fontify-src-code-blocks.html#org4218fd0

I use emacs with a theme (gruvbox) and when I literally use the proposed 
code

    (defface org-block-background
      '((t (:background "#FFFFEA")))
      "Face used for the source block background.")

it does work. But I want to use a color, which is derived (made darker 
or lighter) from another color by an emacs lisp function, so basically I 
want something like this:

    (defvar mycolor (color-lighten-name "navy" 20))
    (defface org-block-background
      '((t (:background mycolor)))
      "Face used for the source block background.")

However, when I try this, I get the following error when debugging:

    Debugger entered--Lisp error: (wrong-type-argument stringp mycolor)
      internal-set-lisp-face-attribute(org-block-background :background 
mycolor #<frame emacs@Machine 0x11e7c30>)
      set-face-attribute(org-block-background #<frame emacs@Machine 
0x11e7c30> :background mycolor)
      [...]

Does someone know why it fails? Is it not possible to use a variable for 
the color there?
What can I do to make it work or to achieve my goal in a different way?

I searched the docs and manuals, but didn't find a solution. I also 
checked the type of the variable 'mycolor' and it is in fact a string 
and 'stringp mycolor' returns t, so I don't understand the error 
message. But my knowledge of elisp is very sketchy, so I might miss 
something very basic.


Thanks
suzee