PCA Regression Problem
First of all, you can project your explanatory variable (continuous) in your first plane (PC1 + PC2). The direction of the arrow (projection) and how far goes from the axis origin will tell you how the points are distributed according to this representation of variables in your factorial plane.

On the other hand, the quick answer is to group your continuous variables into chunks (descretizise your variable into an ordinal one), then you'll have the same plot as the reference.

Furthermore, you could try to color your scatter plot using a color scale (from white to black, red to blue...), and you'll see if there's some kind of progression of your data in that factorial plane according to the continuous variable. 

These three "strategies" actually are showing the same, although the second one is more sensitive to the cuts.

Summarizing:


Project the continuous variable in the factorial plane (what PCA usually does).
Group your continuous variable in groups and plot it (easier way to see although it is sensitive).
Color your scatter plot using an scale (using min and max value of your continuous variable).