Here's the updated code:
```python
fig, ax = plt.subplots()
# Add data: "co2", "relative_temp" as x-y, index as color
ax.scatter(data["co2"], data["relative_temp"], c=data.index)
# Set the x-axis label to "CO2 (ppm)"
ax.set_xlabel("CO2 (ppm)")
# Set the y-axis label to "Relative temperature (C)"
ax.set_ylabel("Relative temperature (C)")
plt.show()
```
We use the `c` keyword argument to pass in the index of the DataFrame as input to color each point according to its date. We then set the x-axis label to "CO2 (ppm)" and the y-axis label to "Relative temperature (C)".