Pronunciation difference between 绿, and 陆。
The initial in 綠 (lü) and 陸 (lu) are the same. You are essentially asking how to pronounce the pinyin ü and u.

Silently, round your lips tightly into a small circle (but don't close it completely). Within the oral cavity, place the tip of your tongue at the very front. Now make some sound from your vocal cords. You are now pronouncing the close front rounded vowel /y/, or the pinyin ü.

Continue to make sound, but now slowly move the tip of your tongue backwards, until you hear the familiar /u/, or the pinyin u. This is the close back rounded vowel.

You may draw reference from other languages that you are familiar with when learning how to pronounce a certain vowel in a new language. (More examples of /y/ and /u/ from Wikipedia)





/y/, ü
/u/, u




English
few [fjyː] (multicultural London)
boot [bu̟ːt]


German
über [ˈyːbɐ] 'over'
Fuß [fuːs] 'foot'


French
tu [t̪y] 'you'
où [u] 'where'




I think the difference in German (ü vs u) is best understood. It is even reflected in its spelling.