Part 1 of Lloyd's weekly bonsai maintenance

Firstly - make a lovely cup of coffee and take a relaxing look at your bonsai.
Secondly remove any yellow or discoloured leaves.
All trees shed their leaves. Old ones will lose the green chlorophyll which shows the leaves true colour, usually yellow. At this point the leaf has no purpose so will dye. It is a constant cycle.
New leaves will replace them during the growing season.
Some leaves maybe be damaged, either by insect damage or fungus infection. This is perfectly normal as all plants, even indoor ones, have these problems.
Just ask my dad about his roses.
With weekly maintenance these problems can be quickly rectified before they become a problem.
Removing the problem leaves will help stop any further damage and help new leaves grow in their place.