Doing that in one pass is likely not an option, since there is no way to independently control the text colour. However colouring the labels with one xline call and the lines with a second xline call is certainly possible —
fig = figure();
ax = axes(fig);
ax.XTick = 1:14;
dy = ["S","S","M","T","W","T","F"];
xline(ax.XTick-1,'-k',repmat(dy,1,fix(max(ax.XTick)/7)), 'LabelOrientation','horiz','Color',[.9 .7 .5]);
xline(ax.XTick-1,'-k')
fig = figure();
ax = axes(fig);
ax.XTick = 1:14;
cm = colormap(turbo(max(ax.XTick))); % Define 'colormap'
dy = ["S","S","M","T","W","T","F"];
dyv = repmat(dy,1,fix(max(ax.XTick)/7)); % 'dyv': Days Vector
arrayfun(@(idx)xline(idx-1,'-k', dyv(idx), 'LabelOrientation','horiz','Color',cm(idx,:)),ax.XTick);
xline(ax.XTick-1,'-k')
The first approach uses the same colour for each day, and the second a different colour for each day. Any colormap will work.
The same cautions apply here, specifically that the elements of ‘cm’ and ‘dyv’ have to be consistent with the value range of ax.XTick.
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