[R] setting specific colours for different groups of points on a scatter plot

[Sarah Kinnings]

Hi,

I have a table with three columns of data, let's call these a, b and  
c. I have produced a scatter plot of a against b, but I would like to  
colour the points according to their corresponding c values. c takes  
the value of either 0,1,2,3 or 4 and so I would like to colour all of  
the points with a c value of 0 one colour, all those with a c value of  
1 another colour etc. When I use the following code, it colours the  
points using default colours, which are not suitable:

data<-read.table('abc',header=T, sep="\t")
attach(data)
library(lattice)
xyplot(a~b, col=c)

I was therefore wondering how to specify my own set of colours for  
each of the c values. Any help would really be appreciated as I have  
spent ages reading through the documentation, but with no luck.

Thanks,

Sarah