How to Set a Quarto ValueBox Background Dynamically: Convert RGB to Hex Instead of CSS Declarations

Learn how to set a Quarto ValueBox background color dynamically: convert RGB to hex, avoid CSS strings, and ensure reliable dashboard styling with Python.

Setting the background color of a Quarto valuebox seems straightforward until you try to drive it dynamically with RGB. If you pass a CSS declaration into the color field, the box won’t pick it up. Here’s how to wire it correctly without fighting the framework.

Problem

You have a Quarto dashboard and want to set the valuebox background with RGB. Writing a CSS-like string into the color property looks tempting but doesn’t work as expected.

---
title: "Sample"
format: dashboard
---
```{python}
#| content: valuebox
#| title: "Sample Box"
dict(value=10, color="background-color: rgb(255,0,0)")
```

What actually goes wrong

The valuebox color input is not a CSS style string. Supplying a declaration like background-color: rgb(255,0,0) is redundant and syntactically off for this field. Using rgb(255,0,0) directly might look reasonable, but it doesn’t yield the expected result here either. The safe path is to feed a color value that the component resolves reliably.

Solution

Convert the RGB triple to a hex color and pass that hex string to the color field. This keeps the value dynamic while matching what the valuebox accepts consistently.

---
title: "Sample"
format: dashboard
---
```{python}
#| content: valuebox
#| title: "Sample Box"
dict(value=10, color='#%02x%02x%02x' % (255, 0, 0))
```

The formatting expression produces #ff0000 for the given RGB values and sets the background correctly.

Why this matters

Dashboards often need to color-code metrics based on data. If the color channel can’t be driven reliably, conditional visuals, thresholds, and alerts become misleading. Using hex ensures predictable rendering without relying on CSS parsing in a field that doesn’t accept style declarations.

Takeaways

When you need dynamic background colors for a valuebox in a Quarto dashboard, don’t pass CSS declarations. Convert RGB to hex and feed the hex string to the color property. This approach keeps the logic simple, works consistently, and avoids surprises during rendering.