Question

Is there a way to assign two different colours according to categorical values in rect graph??

rect(0, yb, Studies.sort,yt,col=("black","lightgray"[Area$"Theory"])

obviously.. this is wrong…..

Area consist of two categorical variables – “Theory” and “Vocational”.

Answer 1

You were close, the following works:

plot(1:10)
mydf <- data.frame( xl =1:5, yb=1:5, xr=2:6, yt=10:6, 
    group=sample( c('A','B'), 5, replace=TRUE) )
with(mydf, rect( xl, yb, xr, yt, col=c('black','grey')[group]) )

It is important that group here be a factor (not just a character vector). However if you use a named vector of the colors (with the names matching the group variable) then it would also work with a character vector.