As RussellB stated, you cannot control what others see in that way. They can choose to use your colors if they want to, but you cannot force them to use your colors. If you want something to look "good" to other people, it's best to stick with default mIRC colors because most people use the default colors. If you use custom colors, you might be mixing colors in an awkward way that doesn't look good when others view them. If you're just displaying things locally, then changing colors to make something look "better" is fine.
