So, in my recent attempt to use a /sci/ meme that I doubt anyone will recognise, I noticed that what I did could be used by people who need to post spoilers.
The two web colours that are used in threads are
color="#e7eaef" (for the dark grey post backgrounds)
and
color="#f0f4f7" (for the light grey ones)
If you use the "change color" function, you can change the colour of the text to whichever of these colours is appropriate, making the spoiler invisible to anyone who doesn't highlight the text. Therefore, when your post is going to have the light background, you'd need
[color=#f0f4f7]This is a spoiler[/color]
And when your post is going to have the dark background
[color=#e7eaef]This is a spoiler[/color]
Note that you must use the # before the colour code.
The results will look like this:
This is a spoiler: Aeris dies and Bridget is a dude. Highlight the text to read it.