You can apply for Hong Kong tourist visa through the visa office of China embassy in Colombo.

The requirements are:
1, Passport
2, Properly completed HK visa application form with one recent passport-size color photograph.
3. Copy of the passport face page and the previous HK visa (if available)
4. Travel itinerary
5. financial certificate
6. transporation arrangements, such as flight ticket.

My wife and I tried to apply for tourist visas to HK at the Chinese Embassy in Colombo but our applications were not accepted. We were told by the ladies at the first counter to make the applications direct to the Immigration Dept in HK. Apparently, even though the official position is that the Embassy is authorised to issue Tourist Visas to HK, the officers at the Embassy refuse to do so unless its a group visa for 5 or more persons. I tried to speak with the Visa Officer but there's no way past the first counter.