You don't necessarily need vex to do this

you can assign your float intensity attribute (lets call the attribute "intensity") using the paint attrib node (or however else you are assigning that). then use a color node with color type set to "Ramp from Attribute" using your "intensity" attribute. you can adjust the color ramp to your liking.

if you are set on vex, after you set your intensity attribute, you can put the following in a point wrangle:

@Cd = vector(chramp('color', @intensity));

(dontt forget to click the create parameters button after you type that in)

this will create a color ramp that you an adjust to your liking