Hello. I read on the medical Transcription rules that a single space was to be used after punctuation at the end of a sentence and after a colon, both of which I always thought was a double space. I just wanted to make sure that I understood this correctly. So, is it a single space after a period before starting a new sentence? and a single space after a colon?

That's an interesting question. When you take a typing class, you learn to always space twice after a period or a colon, single space after a semicolon, etc., to the extent that it is so automatic, it would be difficult to change. However, with the advent of the Internet and html coding, single spaces are part of the programming dynamic so that if you import a document in a standard word processor, the double spaces automatically get changed. People who use the net a lot space that way now, so, bottom line: Do what you normally would do unless you are instructed otherwise by an editor or a client.

Here is what the "rules" say:
Type One Space...
between words
after a comma
after a semicolon
after a period following an initial
after a closing parenthesis
on each side of the x in an expression of dimension, e.g. 4 x 4 type 2 spaces...
after punctuation at the end of a sentence
after a colon except when expressing time or a dilution ratio