gnuplot matrix or plot : display both color and point value

To plot the labels, just use the with labels plotting style. You can use any string and string formatting e.g. with sprintf to set the label:

reset
set autoscale fix
set palette defined (0 'white', 1 'green')
set tics scale 0
unset cbtics
set cblabel 'Score'
unset key
plot 'data.txt' matrix with image,\
     '' matrix using 1:2:(sprintf('%.2f', $3)) with labels font ',16'

The result with the pngcairo terminal and gnuplot 4.6.3 is:

The data file data.txt for this example is:

0.22 0.13 0.54 0.83 0.08
0.98 0.57 0.52 0.24 0.66
0.23 0.68 0.24 0.89 0.76
0.89 0.78 0.69 0.78 0.10
0.24 0.77 0.27 0.28 0.69