Are there any countries where red clothing is frowned upon?

So generally, no, there is almost no country that has an issue with particular colour of clothing of any kind.

HOWEVER, there are two major caveats: sports teams, and gangs.

For example, wearing a La Boca soccer jersey at the La Plata stadium in Buenos Aires could see you in a bit of trouble. And wearing the wrong gang's colours in another gang's neighbourhood in some locales could cause you grief too. So if that was something he or you were worried about, if planning on attending a sports event or checking out some of the sketchier areas of a city, it might be worth knowing about.

However, there are certainly SOME instances of colours being banned. Just earlier this year, Robert Mugabe, President of Zimbabwe banned the colour white (I kid, that's a satirical website, but it shows that it's a ridiculous concept).

Some countries have banned certain natural dyes from being used in food and clothing, because they were derived from substances like arsenic or mercury. Then artificial dyes have some problems too, but that's off-topic.

So then we look at some historical cases where colored clothing has actually been banned. Some of these come under Sumptuary laws, and I'll highlight some below.

The first written Greek law code banned women from wearing embroidered robes and men from wearing gold rings. No other specific colours though.

Ancient Rome limited the number of stripes on your tunic, and only members of high standing could wear purple tunics.

Many other limitations on groups of clothing, courtesans wearing 'flame-coloured' clothing and more can be seen in that list. However, none of them past or present cover

Possibly the only instance where I can find an item of red clothing could be potentially problematic is in recent history, where several countries used a red fez as indication of military status. However due to its rise in tourism, pop culture (Doctor Who famously wore one in his 11th incarnation) this is decidedly no longer an issue. In addition, the British used to be known as Red Coats for their red military clothing, until beaten in the first Boer War, at which point the logic of camouflage won out.

So in conclusion, aside from those who might consider red trousers a fashion faux pas, wearing red trousers should cause no real offence anywhere, unless you've managed to upset a sports fan or gang member. Don't wear a red Liverpool top at the wrong game, for example.

In Thailand, red shirts are associated with United Front for Democracy Against Dictatorship, an opposition group. I don't know how red trousers would go.

As an additional note, during the period of mourning for King Rama IX, people (presumably locals, though I don't know) are encouraged to wear sombre coloured clothing.