Why is Krishna often shown as having blue skin?
It is thought that a demon attempted to kill baby Krishna by giving him poisoned milk. This made the baby turn blue but he did not die and the demon shrivelled up into ashes.
Don't people get upset when coloured paint is thrown at Holi?
People throw powder paint (called "gulal") at each other (yes, even at complete strangers) and no-one seems to mind. The air is often bright with clouds of coloured powder.
You can throw paint at strangers, soak your friends with coloured water while saying "don't be offended, it's Holi", and, unless you're very unlucky, no-one will be upset. (But don't try this outside India!)