Which element does the blue colour represent?
As a planet colour, blue belongs to Shani (Saturn); as an element, it represents the water element.
Related questions
How can I choose the colour that awakens my fortune for my bedroom if I have no horoscope or cannot read it?
Take a colour catalogue and turn its pages with your own eyes: the colours you are drawn to are the ones that awaken your fortune. Use that colour along with the colour of the zone you sleep in, and if possible match it with your horoscope; it usually matches. Sleeping steadily for eight hours in that colour lets its planet act on the whole body.
If Mars as the 10th lord (karmesh) is debilitated, should red be avoided in the logo?
Yes. A debilitated karmesh does not support you, so avoid its colour in the business and the logo, and take the colour of the 11th lord (labhesh) instead.
If a planet's colour (for example Mercury's) cannot be used in its zone, can the colours of its friends or of the zone element's friends be used?
First see what Mercury is in your chart (exalted, bhagyesh or lagnesh, which supports health) and where it sits. If its own colour cannot be used in that zone, use the colours of Mercury's friends, and the colours of the friends of that zone's element must also be taken.
If the colour of a small area in the earth element zone is changed, or an imbalancing item is kept there, does it affect the whole house?
Yes, it affects the whole house. Just as dirt in one room spreads its smell through the air to the other rooms, an imbalance in one zone spreads to the entire house.
Is a dark grey background necessary, for example when making videos?
No, it is not necessary; many videos are made without any dark colour. If Saturn or Rahu is strong in your kundali, dark colours may even suit you. A video is short, so do not worry much about it; think carefully about colours in the office and bedroom, where you spend long hours.
My west-facing house has a lot of grey and it now feels irritating; what should I do?
With Saturn and Rahu in the 8th house in sign 8 (Rahu debilitated), grey should not have been used. Change the wall colour of the largest carpet area; the furniture need not change. Off-white walls are fine, so no need to worry about them.