how do you color the cell in rmarkdown pdf output

I cannot get cellcolor to work in rmarkdown:

---
header-includes:
   -  \usepackage{colortbl} 
   -  \usepackage{color} 

output:
    pdf_document
---

```{r, results="asis"}

library(xtable)
# Your data
tab = data.frame(category = c("A","B","C"), groupA = c(.2,.3,.5), groupB= c(.6,.7,.9))

# Function to cut your data, and assign colour to each range
f <- function(x) cut(x, c(0, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, Inf), 
                      labels=c("green", "red", "blue", "orange", "yellow", "purple", "brown", "white"),
                      include.lowest = FALSE, right = TRUE)

# Apply function to columns: this overwrites your data
tab[c("groupA", "groupB")] <- lapply(tab[c("groupA", "groupB")], function(x)
                                            paste0("\\cellcolor{", f(x), "}", x))
# Sanitise output 
print(xtable(tab), sanitize.text.function = identity)
```

I keep getting this error:

! Undefined control sequence.
l.155 1 & A & \cellcolor

Any ideas what does cellcolor needs to work?