1. Are there Halachik sources for covering the entire knee even when wearing coloured socks or stockings sitting or standing? 2. Are those who are lenient in this area going against Halacha or are there opinions to rely on?

 

1. Yes.

See here at length.

The real question is the other way around: do we have any original sources that indicate that it is OK that the bottom part of the leg not be covered with a skirt? Indeed, historically, women would wear long dresses.

At any rate, the Rebbe writes (לקוטי שיחות יח ע׳ 447) that having the skirt covering the knees is the minimum acceptable level for Jewish women and girls.

2. There are no such opinions in Halacha. Even if many would do so, the Halacha does not change.

In the words of the Tzemach Tzedek (שו״ת צמח צדק אבן העזר קלט):

הנאמר שאם ינהגו לגלות שוקן בימי הקיץ זמן החום יהי׳ מותר שהרי נהגו כן, שתקע הדבר ולא יאמר דכיון שהוא ערוה לא מהני מנהג. ומנהג כזה הוא אותיות גיהנם וכו׳ .