GL-5 Gwoyeu Row -u- Finals 國語羅馬字 ㄨ 行韻母的標調法

In Gwoyeu Luomaatzyh (Gwoyeu Romatzyh, National Romanization), most row -u- finals show tone by changing the medial vowel (u, w, o = 1st, 2nd, 3rd tone), rather than adding a letter so they are printed in green. 

Gwoyeu Luomaatzyh -u- Row Finals (How to Show Tones) 國語羅馬字 -u- 行韻母的標調法

國語羅馬字 ㄨ 行韻母大部分都是靠韻母的改拼來標調 (u、w、o = 一、二、三聲), 因此印成綠色。

3rd and 4th tone syllables that have no initial are spelled with a w- , 

so 勿, 望, 我 = Wuh Wanq Woo (Not Uh Uanq Uoo)

ㄨ行的三、四聲字若沒有聲母, 一律以W-開頭：

勿    望    我 = Wuh Wanq Woo (Uh Uanq Uoo錯)