Specifically to this sentence, there is a rule (though there may be some exceptions) that when an 'O' sound becomes 'U', the following letter gets "דגש חזק". Examples -
צהוב => צהובים (tsahov => tsehubim)
ארוך => ארוכים (arokh=> arukim)
דוב => דובים (dov => dubim)
Also, most Hebrew speakers would use the form "ratuv" => "retuvim" which is grammatically correct as well.
There's a rule, though what makes this complicated is that spoken Modern Hebrew has drifted away from the rule, so you tend to sound a bit like you're trying too hard when you follow all of the rules, almost like if you overenunciate in English and people give you funny looks. The rules here are largely used for official speech, like on news broadcasts.
There's a line in a song by Sarit Hadad "ליבי רטוב". Does this make any sense? (Apart from the fact that it rhymes to the previous line?) http://m.shironet.mako.co.il/artist?type=lyrics&lang=1&prfid=1015&wrkid=18225