Colour message prettifier in Bash

Question

Features:

• messages formatted with clear borders
• errors sent to stderr
• colour handling
• colours can be included in other strings echoed to the screen without having to use the -e option
• the desired colour can be either a string or a function
• colours can be issued as commands before echoing text

Limitations:

• If the message is longer than the width of the terminal, the output will be strange
• If the message contains newlines, the output will be strange
• Non-colour formatting functions can be passed in
• Non-colour strings can be passed in
• The terminal is assumed to always use the default colours and resets back after each message

Questions:

• Is there a simpler way to get a string of spaces the same length as the message passed in?
• Is there a simpler way to get the output of a function if it exists, or a default value if it doesn't exist?
• My preferred order of function execution is top-to-bottom, but BASH seems to do a single pass, so I've got bottom-to-top because my code immediately calls the functions above it. Is there a standard order of function declarations/definitions in BASH?

I'm very experienced in other languages, but I'm pretty new to BASH, so I could be making newbie mistakes. All feedback gratefully received! (unless it's mean!)

#!/bin/bash

fg_black="\e[30m"
fg_white="\e[37m"
bg_red="\e[41m"
bg_blue="\e[44m"
bg_purple="\e[45m"
black_on_red() { echo -en "$fg_black$bg_red"; }
white_on_blue() { echo -en "$fg_white$bg_blue"; }
black_on_purple() { echo -en "$fg_black$bg_purple"; }
reset_colours() { echo -en "\e[0m"; }

function_or_value() {
        func=$(declare -F "$1" 2>/dev/null)
        local value=$($func)
        echo -n "${value:-$1}"
}

colour_message() {
        local colour="$(function_or_value "$1")"
        local message=$2
        echo "$colour  ${message//[^ ]/ }  $(reset_colours)"
        echo "$colour  ${message}  $(reset_colours)"
        echo "$colour  ${message//[^ ]/ }  $(reset_colours)"
}

colour_error_message() {
        local colour=$1
        local message=$2
        colour_message >&2 "$colour" "$message"
}

colour_error_message "$(black_on_red)" "Error!!!"
colour_message white_on_blue "No Error!!!"

Answer 1

fg_black="\e[30m"
fg_white="\e[37m"
bg_red="\e[41m"
bg_blue="\e[44m"
bg_purple="\e[45m"

These control codes are terminal-specific. Prefer to use tput to create the appropriate codes for the actual $TERM being used.

Answer 2

Make a string str full of spaces with the length of variable a:

printf -v str "%*s" ${#a} " "
# or
str="${a//[^ ]/ }*"

I don't like the feature that you can have 2 different kind of parameters (string or function name) for setting a color. In the future you might want to enhance your function (make it split up the sting to a fg and bd color, using _on_ te keep them apart) or limit the accepted color combinations.
However, if you really want to do so, you can try

[[ $(type -t "$1") == function ]] && "$1" || echo "$1"
# or
[[ "$1" =~ "_on_" ]]  && "$1" || echo "$1"

https://google.github.io/styleguide/shellguide.html#function-location suggests the order you use. I never use the suggestion of a main() fuction.
You might want to move all your functions to a different file that you can include with source.