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.
Related questions
Which element does the blue colour represent?
As a planet colour, blue belongs to Shani (Saturn); as an element, it represents the water element.
As an interior designer, clients choose their own colours and I cannot check their planets; should I give a remedy or try to convince them?
Do not try to convince anyone. People mostly like the colours that match their own planets. If a client chooses an anti-element colour and their kundli allows it, never stop them; it will bring them benefit.
Can a rectangular shape be used in the North?
It is better avoided. In the North, choose a half-moon type shape as far as possible.
Can dark blue be used in the North?
Green is best for the North; blue is for the North-East.
Drainage is listed under the water element. Is a drain line in the water zone negative?
Underground water movement 1.5 to 2.5 feet below the surface is not treated as directional, just as groundwater flows in every direction under the land; it becomes a dosh only when water is drawn up. So the drainage pattern itself is not a dosh. But a drain inspection chamber built at a wrong place and left open causes trouble, and dirty water flowing anti-clockwise has a considerable negative effect.
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.