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Your care and especially your English are good. I would not use peroxide on roots. It can damage them. We say "shade cloth."
The scale insects are difficult to control. They live in your environment so there is no point to using poisons. You can kill tthe adults by spraying with 70% alcohol or with dish soap 5ml in a liter of water. They will return.
Vandas with magnesium deficiency get purplish coloration during cooler weather. Rhynchostylis and Cattleyas have it normally. If that is a Vanda it's probably magnesium deficiency. If you can get epsom salts, put a spoonful in a liter of water and use that for watering. It can take months to correct.
The yellow spots are likely damage or fungus around insect bites. Fungal issues are common in humid tropical climates. If it's not seriously affecting growth I would ignore it. Not every fungus is susceptible to every fungicide, so you need to find out which fungus you have before choosing a fungicide effective on that fungus. A local orchid society may be helpful. The Motes Orchids Web site lets you sign up for a monthly newsletter that covers growing orchids in humid tropical areas. Motes discusses fungicide use.
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