This might be the easiest way to incorporate unexpected color into an outfit. Either buy a less expensive watch with a bright strap, or invest in a watch where you can change out the straps. Go for a color, like yellow or red, that is bright and offsets the rest of the pieces in your outfit.
The casual oxford is a shoe that's nice to keep around to dress up a casual outfit, especially in cooler weather. If you're not in the mood for a boat shoe or a loafer, these are a great sneaker alternative. Take your oxfords up a notch with a colored sole and/or laces. Like a watch, this is a pretty subtle way to wear color, but a pair of shoes are a little more of an investment than a watch or watch strap. If you're an oxford person or looking for a way to amp up your look instantly, this investment is worth it.
This one is a little trickier because in the case of glasses, you wear them every day unless you've spent the money to have multiple pairs. But colored rims are a quick way to trick the passerby into thinking that you are ultra-stylish. It's nice because glasses don't have to match what you're wearing and are kind of separate from your attire as a whole, so if you see some frames you like in a color you don't necessarily wear a ton, it's not something you have to be really concerned about. This purchase is more of a commitment, but if you're ready for it, it will really pay off.
Possibly the safest way to easily add color, these can be cheap and very low-commitment. The only problem is that you can only do this in certain dress codes. However, suits are often the hardest item to make interesting, so go for patterns and colors so you can easily mix up your suit look, especially if you wear suits a lot and can't invest in a ton of different suits.