Try this:
% Define the two colormaps.
cmap1 = hot(15)
cmap2 = winter(15)
% Combine them into one tall colormap.
combinedColorMap = [cmap1; cmap2]
% Pick 15 rows at random.
randomRows = randi(size(combinedColorMap, 1), [15, 1])
% Extract the rows from the combined color map.
randomColors = combinedColorMap(randomRows, :)