Delete a row in R based on next row condition

You don't need loops for this. You can do it with a single line in base R by combining a grepl with a lagged grepl

df1[!c(head(grepl(":", df1$a), -1) & tail(grepl(":", df1$a), -1), FALSE),]
#>   b        a
#> 1 1   value1
#> 2 2   value2
#> 4 4 value2:b
#> 5 5   value3

Here is a dplyr solution.

library(dplyr)

df1 %>%
  mutate(flag = grepl(":", a),
         flag = cumsum(flag)*flag,
         flag = lead(flag, default = 0)) %>%
  filter(flag != 2) %>%
  dplyr::select(-flag)
#  b        a
#1 1   value1
#2 2   value2
#3 4 value2:b
#4 5   value3