How can I concatinate strings that have a specific pattern with the previous string in character vector?

I have a character vector listed below.

vec <- c(
  'red yellow orange', 'green white', 'blue', 'orange purple', 'green brown', 'red'
)

I would like to paste blue and red in front of the string that precedes them. Here is my expected output.

[1] "red yellow orange" "blue green white" "blue" "orange purple"  "red green brown"  "red" 

>Solution :

with ifelse and head_tail

c(ifelse(
   grepl("^red|blue$", tail(vec,-1)), 
   paste(tail(vec,-1), head(vec,-1)), 
   head(vec, -1)), tail(vec,1))