If the legend displays the color scale accurately, then it is an effective legend. if your data is continuous interval level data then a color map is suitable.

if you have continuous data but need a limited set of discrete shades of color (out of accessibility requirements, for example) you may consider binning the data into buckets of, say, 0%, 25%, 50%, 75% and 100%, and map these to 5 different shades of a single color. You should be aware of drawbacks to this method in certain scenarios however, because outside of your visualization, these arbitrary cutoffs may not be meaningful to users.

But if the instruction is to simply include a legend, there are many types of legends to meet this standard and the one you choose should reflect the scale of whatever kind of data you are displaying.