After spending many sessions reviewing projects I'm curious to know if you met what the requirement was and was their enough information before you did the design. In no way am I trying to be rude but we are only receiving one side of the spectrum and if he's asking for more colors I wonder was this an "open" design.
I struggle with someone higher up the food chain, who has no
experience in graphic design
Then communicate, communicate, and again communicate. If they have no clue then educate them, NICELY, dont get offended, dont become rude and explain to them about the design. I think DA01 put it nicely when he said let the "design speak". In time you will either hate going into work or lose the passion. Tons of people in different fields experience the same issues and it is very common in the corporate world where you have an educated person that understands what to do and then you have someone higher up that doesn't understand anything about how a system work and wants to put their spin on it or put their two cents in. I battle it everyday in the web realm but I enjoy what I do and I know I'll have to educate on why something was done (think I vented).
doesn't know current trends
Sorry I dislike this mentality because good design is good design. If you are following "trends" you are just riding the bandwagon of over played designs.. Case in point look at flat web design. Also, if you follow "trends" you will be re-doing your work about every two to five years depending on the trend.
looking at my work and giving me vague instructions such as - "give
it some more colour" or "jazz it up a bit"
Have you reached out and asked for examples in the brief of the project or asked him now what colors he likes? As said above is your design open or is their a requirement? You have to treat your boss and the people in your company just like you would do when freelancing and ask. If they cant show you, ask for an example, are they trying to mimic another project even? Is the design supposed to be a sales run? Don't go into it offensively, stop, take a break, and try to understand what they want and see. I've had one instance in a web design project where I had to physically walk in with an iPad and about 25 images of different sites just to figure out what the client wanted and it turned out what we thought was completely different than what we were told.
I did a nice black on yellow advertisement which utilised nice
typography and a visual hierarchy, was well put together with the 2
colours. His comment was "give it some more colour, maybe some
bubbles to make it a bit flashier".
It sounds like an awesome design, but as said already in the answer, did you meet the requirement? Was there a color palette requested? Did you ask for an explanation on "more colour"? Is he talking about more shadowing, highlights, physically adding another color like orange?
Problem is "more colour" ruins the effect in my opinion. Basically he
seems to think "more is more" and I think his suggestions are just
distracting from the main message of the piece. Basically my problem
is I feel I'm taking instructions from someone who knows less about
design than I do.
I think you're taking the design to heart (which is not bad) and maybe are getting burned at not receiving designs that require no revisions. This sounds differently then what Im trying to say. Its great that you're passionate about your design BUT you must give in on some change from your boss. You shouldn't go into a design with this is the way and Im always right mentality. I say that because how do you know if he thinks more is more and that is what he wants? If he said he wanted more color then there has to be a reason behind it. Find out that reason and see if you can "jointly" come to a decision on what the project is to be.
How do you deal with this sort of thing? My thought is get him to tell
me exactly what he wants and then just do it, but what he does it he
gives really vague instructions like "spruice it up" or something, and
then I have to think of a way to make my design worse all on my own.
This is common in what many designers face day in and day out. People that cannot comprehend a creative mindset need more communication and you will, at times, find yourself asking the same question four times till you can pull the information from your boss or colleagues. I would suggest, just based on the question format, take a breather, stop and think why they are wanting this, was the brief incomplete, did I not get enough information, is the intended project making the boss think it mimics an existing design?