In versions prior to R2019b, you can always set the axes property without the colororder() function.
% some fake data
x = linspace(0,1,100);
y = bsxfun(@plus,rand(18,100),(1:18).'); % i'm assuming we're in an older version
% set the axes colororder
CT = jet(18); % not that this is a good choice
set(gca,'colororder',CT);
% plot things
hold on
plot(x,y)
Of course, like @Bjorn Gustavsson says, unless you do something beyond this (e.g. using a carefully crafted map or using other line properties), putting 18 curves in the same plot can be a readability problem, especially if they overlap or the results need to be printed. For example:
clf
% exact same colors, but data series don't have sequential offsets
x = linspace(0,1,100);
os = (1:18).';
y = bsxfun(@plus,rand(18,100),os(randperm(18)));
% set the axes colororder
CT = jet(18); % not that this is a good choice
set(gca,'colororder',CT);
hold on
hp = plot(x,y);
% which curve is data 4?
legend(hp,'location','eastoutside')
See also:
