Why are the colors of some elements toned down?

If the colors you've chosen are not fully saturated, the elements were likely created in wireframe mode. Wireframe elements have toned down, more transparent colors by design, so they won't be fully saturated. This helps keep wireframes semantic (primarily about the content and layout).

Note: Sections also use a less saturated version of the colors in your chosen theme.

If you need more vibrant colours, you could add shapes from the flowchart range - they will be true to the color you've chosen from the palette. Here's an example of color differences between flowchart shapes and wireframe elements: