Categorical data in Kmeans I need to perform clustering in a given dataset.
There are atrributes with numerical as well as categorical values.

What is the best way to convert categorical to numeric value ?

as an example one field is colour and the values are red, green, blue
so can I assign a  mapping like:

red : 1, green : 2, blue : 3 or
red : 11, green : 25, blue : 30 

and if I provide a mapping like this will this affect the Euclidian distance for clustering ?

or is there any other way ?
Converting the categorical data into numerical data isn't really meaningful. Different mappings will give you different solutions.  

There is an extension of K-means algorithm for categorical data called k-modes. You can read about K-modes in detail here. This article explains the difference between K-modes and converting data into numeric vectors and then running K-means.