Trouble with color scale of contour plots


I’m attempting to make a series of models for soil moisture profiles and I have a series of treatments and am trying to display the change in soil moisture over time. I am attempting to do this using contour plots. I have been able to make individual plots for each each treatment at each time, but they each have a separate color scale within the same color palette (ie. a value of 10 would appear blue in some and red in others). This is obviously a problem when trying to compare plots and construct multiple subplots to visualize change over time. Is there a way to lock the color scale so I have a consistent representation of those values between treatments and subplots?

Here is my code thus far for an individual plot and my attempt at combining them into a subplot

on1<-plot_ly(data=idw.output, x=~Width, y=~Depth, z=~Moisture, type=“contour”,colors=“Spectral”, reversescale=TRUE) ###To make a single plot

H4<- subplot(on1, on2, on3, on4, on5, on6, on7, on8, on9,nrows = 3, margin = .01, shareX = TRUE, shareY = TRUE)###subplot displaying those plots together for each treatment

Any help or suggestions are greatly appreciated.