Be loud but don't shout in the user's face, you want the impact of your UI to be exciting and meaningful and not aesthetic noise.

Loud noises can immediately get people's attention, and so can UI elements. Therefore, you want to make your more important elements the focal point. The key here is to have fewer elements so that the player doesn't feel overwhelmed. For example, in a game, you may want the health bar to be the main item of focus. Therefore, place it somewhere where it will be noticed and not in the corner in the player's peripheral view. One way to achieve this is to have the UI elements contrast the environment, ideally within the same color palette so that they stand out, but not draw so much attention that they are distracting. A great website to create great color schemes is Adobe Color CC (https://color.adobe.com/). An example of it can be seen in the following screenshot:

Color is a great way to convey information to players for various reasons. Just like a red traffic light means stop and green means go, the same approach can convey something immediately to a player with little effort. A common example of this in games is with a health bar. In general, when it is full, it is green. As the player engages in combat and takes damage, it begins to deplete and turn red, therefore instantly telling the player they're at risk of dying.