Btw Thomas, how do you highlight someone's quote in purple? Thx 
If you would like to quote someone's post, indicating that you are replying to their post or referring to their post, you would click on the "Quote" button at the top right corner of the post to which you would like to reply. The message window in which you would compose your post is then prefilled with the quoted post.
There are other buttons that have other functions that you may want to experiment with, as there are with other types of message boards or forums.
Like DCJ001 said, if you just want to quote someone else's post you click the QUOTE button and it will do it automatically for you.
However, let's say you want to answer multiple statements within that one post. Then here's what you do:
Btw Thomas, how do you highlight someone's quote in purple? Thx 
Is coded in UBB below
[quote]Btw Thomas, how do you highlight someone's quote in purple? Thx :)[/quote]
Use the quote tags as to quote any part of a post: [quote] INSERT WORD/SENTENCE HERE [/quote]
If you want to make your reply stand out inside of a quote you can use different color text by the code below like this:
Btw Thomas, how do you highlight someone's quote in purple? Thx 
See the code below
[quote]Btw Thomas, how do you highlight someone's quote in purple? Thx :)
[color=blue]See the code below[/color][/quote]
Do a quick search on any search engine for UBB Code and you'll be able to use it on 95% of all forums. Some forums are coded slightly different than others.
Other simple codes are:
BOLD TEXT[b]BOLD TEXT[/b]
ITALICS[i]ITALICS[/i]
UNDERLINE[u]UNDERLINE[/u]
STRIKE[s]STRIKE[/s]