See links from
for all the properties of the text object and examples of using them.
...want it centered...
'horizontalAlignment','center
...and diamonds and hearts to be red...
For the latter you'll have to split the text strings to write the black suits and red suits individually.
Or, you can get more fancier and use TeX and the \color and other directives.
See the section in documentation on Adding Text Annotations to Graphs in the Graphics chapter for lots of examples and the full documentation story.
You can just try the following for starters
text(0.5,0.5,'Clubs','horizontal','center','color','k')
text(0.5,0.45,'Hearts','horizontal','center','color','r')
or
txtstr(1)={'\color{black} Clubs '};
txtstr(2)={'\color{red} Hearts '}
text(0.5,0.5,txtstr,'interpreter','tex','horizonatalalign','center')
Obviously you'll want to build the strings for the hands dynamically.