Big text in the middle of the screen, a smaller line under it, and the line above the hotbar. Type the text, colour it with & codes, and copy the /title commands for your version.
Commands, in this order
How to write the text
&a to &f and &0 to &9 are the 16 colours. 7FF99 is any hex colour.
&l bold, &o italic, &n underline, &m strike, &k scrambled, &r back to the line's colour.
Good to know
Run the commands in the order shown. The subtitle only appears when a title is shown, so the title command comes last. With an empty title box the tool sends an empty title for that.
The times command is left out when you keep the game's own times (0.5 s in, 3.5 s stay, 1 s out). The game counts in ticks, 20 a second; the tool converts for you.
The times stay set for that player until they change again or the player rejoins.
The action bar has its own short time and ignores the fade times.
The way text is written in commands changed in 1.21.5. Pick your version, or the command is refused.
For a command block chain or a datapack function, untick "Start with /".
Something wrong or missing here? Tell me, requests decide what I add next. More tools: all tools.