When trichomes start appearing?

I don't know your strains maturity rate off hand but some are done in 55days. Some discoloration is normal and sugary tricomes are normal in 4th week of flower. My northern light had them beginning of 3rd week. BLUF: as long as the plant looks otherwise healthy, dont worry about it. Also if your temo hits the 60s it will cause leaves to do all kinds of colors and make a purple bud darker

Turning brown at day 28 is not normal for most strains. Either caused by a fan too close, man handling the buds or the most common issue fertilisation by a male or hermaphrodite. If they are fem seeds chances are there is a ball sac or banana that you have missed. You need to go across the plant and any other plants sharing the space and check for male parts. Don’t forget there are some strains that show red hairs as early as this but they are not common so don’t despair just yet buddy👍