"However, there are other long names available: lightblue, darkblue, darkgreen, purple, orange, gray, etc. "
How are you using those 'other long names'? Did you perhaps download an add-on that defines functions by those names to return an RGB value? You can check that using the which function. gray is a function that returns a colormap matrix, but I don't believe the others exist.
>> plot(1:10, 1:10, 'Color' ,'lightblue')
Error using plot
Specified character vector is an invalid color value.
You can define a variable or a function named lightblue and then use it, but I wouldn't call that an "available color name".
lightblue = [0.8 0.8 1];
plot(1:10, 1:10, 'Color', lightblue)