It can be tempting to splash on the colours in a slide design, especially if you have a colour palette with 3 or more colours. The result: highly colourful slides that still look very “PowerPoint”. Why does the work of a professional designer not look like PowerPoint?
The secret: only use colour or highly contrasting greys (dark on a white background, light on a dark background) when you want an object to pop out. Use one base colour frequently, and reserve all the others for accents. Use light shades of grey, rather than filling a big slide object with a very dark grey. Consider using dark grey for your font colour instead of pitch black.