This would be a fine example of spam!!!!
Like the others have said above, there is nothing wrong with just plain black and white text. There are simple things you can do to make your posts more appealing without necessarily adding colors. For example, I sometimes hit enter at the end of sentences so that they line up neatly, instead of having half a sentence down on the next line. You can also break your posts into paragraphs properly, rather than just dragging on for an entire page with no white space.
However, different colors and font settings could be appropriate. For example:
Google will penalize you if you... rabble rabble.
Google will not penalize you if you... rabble rabble.
Be very careful because this action could result in permanent damage.
Note: This is a great use of bold text.
Note: This is not a great use of bold text.
What do you think?
Sizes can also help convey ideas. They can also be used to hide things.
This is a great offer!
(it's not really a great offer, this is a scam.)
100% Iron-Clad Money Back Guarantee!
You can make a million dollars right now without any effort.
(Disclaimer: Money back guarantee applies only to purchases between this and that and other tricky stuff goes here.)