Simplest way to change color of heatmap

[mcp292]

MWE

import altair as alt
import numpy as np
import pandas as pd

# Compute x^2 + y^2 across a 2D grid
x, y = np.meshgrid(range(-5, 5), range(-5, 5))
z = x ** 2 + y ** 2

# Convert this grid to columnar data expected by Altair
source = pd.DataFrame({'x': x.ravel(),
                     'y': y.ravel(),
                     'z': z.ravel()})

alt.Chart(source).mark_rect().encode(
    x='x:O',
    y='y:O',
    color='z:Q'
)

Source.

Question

What is the simplest way to change the color of this heatmap?

Is there, for example, a way to tell it I want the max color to be "red", and have it interpolate from there?

Related

The answer provided here is pretty involved.

[mattijn]

See the Altair docs here: https://altair-viz.github.io/user_guide/customization.html#customizing-colors
For the available color scales, see here: https://vega.github.io/vega/docs/schemes/

[mcp292]

Outstanding thank you! color=alt.Color('z:Q', scale=alt.Scale(scheme="reds")) did it for me!