Dear Punjabi Veer,
Please make the correct use of thread title prefixes. It misleads members.
About your question "Technically what should be the colour of a dish antenna?", the answer is ideally it should be white or light shades of a colour.
Although there is no direct relation between colour of a dish antenna and signal strength, using dark / black colour will absorb the heat from the sun and may cause some slight issues since we know that things expand on heating. Also LNB may get damaged due to the heat radiated on it.