To underline a single word in a sentence:
Type Control-U
Type in the word
Type Control-U again
Only the text that is enclosed by the start and end codes will be affected. You can use this method with all of the other control codes.
The Control-K control code is slightly different because it allows you to specify a colour number. To colour a single word in a sentence:
Type Control-K
Type a number between 0 and 15
Type the word
Type Control-K again
If you also want to change the background colour of a word, you would need to type two numbers separated by a comma instead of just one number. The first number is the text colour, the second number is the background colour. The colours range from 0 to 15, the index is in the previous section.
You can enclose text in multiple control codes, so for example you could have a bold, underlined, and coloured word.