If you're specifying the counts, could you use bar instead of histogram? bar has a CData property for this purpose. The limits will appear slightly different but other than that it's pretty similar.
Notes:
- I called categorical with (x,x) to ensure the categories are plotted in the same order that they were specified (otherwise bar will place them in alphabetical order).
- You'll need to specify FaceColor as 'flat' to make bar use CData
- I used validatecolor to convert the colors to an RGB matrix
x = ["Fossil" "Biomass" "Geothermal" "Hydro" "Solar" "Wind" "Other" ];
C = categorical(x,x);
y = sum(rand(7));
nexttile;
h = histogram("Categories", C, "BinCounts", y);
nexttile
clrs=validatecolor(["red" "cyan" "black" "blue" "yellow" "green" "magenta"],'multiple');
b = bar(C,y,'FaceColor','flat','CData',clrs,'BarWidth',.9);
