My system for creating interesting decor is to layer contrasting colors, textures and/or materials.
Mix different patterns in similar color ways. If you aren’t confident picking patterns, then select coordinating colors with a solid, stripe, and pattern. Perfect example would be on a bed or sofa with decorative throw pillows in a solid, stripe, and pattern. On the image below, I have I used monochromatic color palette with different patterns.
Contrasting materials such as wood, metal, stone, and glass layered is a great way to contract with furniture. One of my favorite things to do is add a painted wood tray on top of a glass or stone table top to layer color and texture. Or the other way around with a metal or stone tray on top of a wood table or console. Here, the painted grey night stand is topped with a painted red tray.
Texture is another way to create interesting contrast. Monochromatic color palettes are perfect for texture contrasts. For example, pairing a woven fabric with leather or vinyl, or a high gloss tile with a matte tile are way to create interest in same color palettes. The backsplash in this image is all the same color, but layered with different sizes and textures for interest.
Once you get the concept of layering contrasts to create interest, it becomes easier to do, and you develop and eye for what works and what doesn’t. In kitchen below, I have layered off white cabinets on top of a sea of blue glass tile.