Try this Source Text expression:
letterColor = [1,0,0,1]; // Red
numberColor = [0,1,0,1]; // Green
symbolColor = [0,0,1,1]; // Blue
myExpr = "style";
for (i = 0; i < value.length; i++){
c = value[i];
if (c.match(/[A-Za-z]/)) {
myExpr += ".setFillColor(letterColor," + i + ",1)";
} else if (c.match(/[0-9]/)) {
myExpr += ".setFillColor(numberColor," + i + ",1)";
} else {
myExpr += ".setFillColor(symbolColor," + i + ",1)";
}
}
eval(myExpr)