How can I change the color(gray) of the CheckBox and RadioButtonList when it is Disabled?

No way. For example, IE lets you style some properties of check boxes but
not the color of the mark. It's actually questionable whether that color
is describable in CSS terms at all - is it really color of the _content_
of the element? After all, <input> elements are empty (i.e. cannot have
any content) in HTML terms.

You might use an image of a checkbox, or you could construct a
checkbox-like appearance by using a element with no-break space or
a check mark character as the only content and with a suitable border.
But there's no guarantee that these would correspond to the appearance of
a checkbox in the user's browser. Besides, the check mark character
is not present in several commonly used fonts.

Why would you use a disabled checkbox in the first place? It's basically
useful only in the case of a page where the content is changed via
client-side scripting, so that a checkbox can be disabled or normal
depending on the state of the page. But in that case, it should look very
much like disabled when it is.

Let me guess... I've read about some rules and regulations that require
that when a form is echoed for confirmation, it must look exactly like
the original form, but you naturally don't want to let users change the
content. The problem has been created by rather artificial rules, and
there is no reliable solution, but I would consider creating a normal form
with normally changeable fields _but_ check in the server-side script that
nothing was changed (which is a useful check, for data security, anyway).