You're on the right track that you need to convert your scattered data into gridded data. A good tool for that job is the TriScatteredInterp class.
% Some data
lat = rand(300, 1);
lon = rand(300, 1);
ptrend = peaks(lat, lon);
% Make a TriScatteredInterp object
F = TriScatteredInterp(lat,lon,ptrend);
% Make a grid of lats and lons, based on the min and max of the original vectors
[lat_grid, lon_grid] = meshgrid(linspace(min(lat), max(lat), 50), linspace(min(lon), max(lon), 50));
% Do the interpolation
ptrend_grid = F(lat_grid, lon_grid);
% Now PCOLOR will accept the gridded data
pcolor(lat_grid, lon_grid, ptrend_grid);