Bingowings said:
Doctor Who never had a problem finding actors of African ancestry because of the influx of West Indian people into the country around the time. Similarly finding actors originating from the Indian subcontinent was also not that difficult.
It became a bit of a stretch casting lots of actors from the Orient.
So while a genuine oriental actors where cast in Mind of Evil when it came to The Talons of Weng-Chiang (which is practically banned in some countries) or Marco Polo the available actors had to be supplemented with white actors in yellow face.
One of the Doctor Who DVDs I watched recently (The Mutants) had an interesting documentary (Narrated by Noel Clarke aka Mickey) called 'Race Against - Time' (LOL) that looked into percieved "rascist" casting on Doctor Who during the 70s. While early 60s Who had colour-blind casting of roles (And great roles for women too for that matter) it also would cast white actors in yellow and brown face on rare occasions during the 1970s. Such as the examples you give and Cho-Je from 'Planet of the Spiders' comes to mind too...

The writers and producers interviewed said there weren't any actors easily available but I find that a bit hard to believe (And many of those interviewed found it hard to believe too). But given the progressive politics of the Who creative team during the 70s, it's inconceivable that it was done with any ill will. It was just a different time as you say, now of course, you wouldn't dream of casting a white actor, you'd just look a lot harder for the correct person.
One theory the documentary put forward was that colour-blind casting across Brtish TV (Including Who) was the norm in the 60s. But after Enoch Powell's notorious so-called "Rivers of blood" speech in 1968, racial tensions were much increased and TV producers became unintentionaly less likely to hire multi-ethnic actors for roles, unless it was a specificly non-white character.
Also, 'Star Trek: TOS' can be jaw-droppingly sexist despite it's barrier-busting reputation. But compared to other shows of the time it really was a leap forward and so was Doctor Who.