Standard keyboarding practice on typesetting machines, however, has always been to adjust the space between words evenly to justify the copy. Typeset copy is generally in a compensatory font. There is extra white space between sentences in compensatory fonts, but this white space is supplied naturally rather than mechanically. Because periods are such small punctuation marks, they leave white space in the very space they occupy. Typesetters, thus, ordinarily leave only one space after a period, reasoning that the period itself leaves the extra space to balance the copy and indicate the end of a sentence.
Colons are actually medial-sentence punctuation.
They occupy more vertical space than periods, but logically need less space than would be necessary after the end of a sentence. Computer-generated copy is usually printed in a compensatory font. Recommended keyboarding practice thus follows typesetting rules rather than typewriter rules. I can recall reading about these standards in several different places. One was a list of suggestions for making one's word-processed copy look typeset.