You can pass a string this way. Try below code
string getColourin = "Red";
String requiredId = driver.FindElement(By.XPath("//option[@selected='selected' and .='" + getColourin +"']/..")).GetAttribute("id");
OR
using string.Format
string xpathBefore = "//option[@selected='selected' and .='{0}']/..";
string getColourin = "Red";
string finalXpath = string.Format(xpathBefore, getColourin);
String requiredId = driver.FindElement(By.XPath(finalXpath)).GetAttribute("id");
Hi brother, could you please share me this answer. link @Muzzamil
@bydukaan I hv answered long back. Please chk there