I've been coming to Thailand regularly for the past 8 years and have never felt or experienced any discrimination based on colour by immigration, or any other Thai officials. Always allowed in without any problems on SETVs and visa exemptions ... so far.
I am currently on a Tourist visa for which I applied to the London embassy by post. Came back by return post, as it has always done over the past several years. Takes 2-3 days. Things could have changed...
Yes, I was caught out once. I had assumed a day was 24 hours! Not in Thailand, a day can be only 5 mins, as you say. I was out by a day and overstayed a day. Only got a finger-wagging on the way out though.