SaiSan Hi,
this not an question about ISSM but about MATLAB π... I think you can do something like that:
>> field = zeros(md.mesh.numberofvertices,1);
>> pos=find(md.geometry.thickness<1); field(pos)=1;
>> pos=find(md.geometry.thickness>1 & md.geometry.thickness<10); field(pos)=2;
>> pos=find(md.geometry.thickness>10 & md.geometry.thickness<100); field(pos)=3;
>> pos=find(md.geometry.thickness>100); field(pos)=4;
>> plotmodel(md,'data',field,'colormap',jet(4))
>> colorbar('XTickLabel',{'H<1','1<H<10','10<H<100','H>100'});
Best
Mathieu