When you were in preschool or kindergarten, your teacher should’ve explained capital and minuscule (uppercase and lowercase, respectively) letters to you. Now, for me, this was before technology was what it is today. But, based on general etiquette, you follow the rules as prescribed by the rule makers. Anything that deviates from the standard means one of three things to me: 1) You never learned it, 2) You are too lazy to follow the rules, or 3) You are breaking the rules to make a point.
Now, within the general body of, say, an email to a colleague, typing a sentence in all capital letters, such as “WHY DIDN’T YOU FOLLOW THE INSTRUCTIONS I SENT YOU?” is unprofessional at best, hostile at worse. Out of the three reasons for deviating that I gave before, I would like to assume that you are evoking reason number 3, breaking a rule to make a point, point being that you are upset. But there are better ways to handle the problems you have with your colleague.
Although many of us love hiding behind the cloak of a darkness that the internet provides, there are more constructive ways to get your point across. I am partial to face-to-face interactions, such as a meeting over coffee. Because so much of communication is nonverbal, whereas you may be calm and accidentally hit the caps lock button while listening to music and talking to your coworker, your request for your coworker to “PLEASE COME TO MY OFFICE WHEN YOU GET A CHANCE. I NEED TO TALK TO YOU ASAP” IS RECEIVED IN A DEFENSIVE MANNER (see how easy it is to forget to turn off the caps lock?).
I could go on and on about effective ways to communicate (and I will in other posts) but right now, just make sure not to look like you’re yelling even if you want to at that very moment. It makes you look overly emotional and underly professional (kind of like the word underly but it just worked at that moment).
Make professional development a priority.