Understand colors of variables in Wolfram Language?

Questions
1

Suppose that I have an code which goes like this:

yp = 2;
y_p= 2;
a = x + yp + y_p;

Please put y with subscript p, NOT Subscript[y, p].

Usually, once the variable is defined it will be of blue color and once it executed (by pressing Shift+ Enter) it will change color to black. But, you can see that x and yp change to black, but Subscript[y, p] still remain in the same color. Why is this so? Is it default in Mathematica that if you use the subscripts as variable, the color will not change after execution? I am attaching the Mathematica file for your references.

Thanks in advance…

2

Underlines are not allowed in variable names in Wolfram Language. I know that this is legal in other languages, and even a common idiom, but the underscore has a special meaning in Mathematica.

If you don’t like the typesetting construct Subscript[], then you can make up your own convention, as long as it does not involve underscores.

3

Using underscores does indeed not work; variables can not include underscores and can not start with a number. Also do not use Subscript or Superscript for variables, these should be used only as a typographical construct. Sooner or later you will get into trouble with those…

4

Thanks Sander Huisman and George Woodrow III for that answer.

I can understand that if variables is starting with number, then it cannot be evaluated. But in my case, it is getting evaluated!

sreeraj t