Minecraft Command trouble - Why won't the command work?

If you want to define the colour of a title in Minecraft, you will need to include both the text and the colour inside of the curly braces.

In your case, the command would look like:

/title @a title {"text":"Flower Shop","color":"dark_red"}

In this command, "text" is used to define the title shown on the screen, and "color" defines the colour of the text.

The same command will work to define a subtitle if you change /title @a title to /title @a subtitle, but you will need to run the subtitle command before the title command, as they will appear together.