Maybe I'm misundertanding your issue but just to be sure, Do you mean that inside your colors variable you have text values like "c+1" and you want to convert them to 1?
If that is the case then you can do it like this
library(dplyr)
library(stringr)
# Sample data on a copy/paste friendly format, replace whit your own data
addition_colors <- data.frame(
stringsAsFactors = FALSE,
colors = c("c+2", "c+4", "c+7")
)
addition_colors %>%
mutate(colors_calc = as.numeric(str_extract(colors, "\\d+$")))
#> colors colors_calc
#> 1 c+2 2
#> 2 c+4 4
#> 3 c+7 7
Created on 2020-10-16 by the reprex package (v0.3.0)
If this doesn't solve your problem, please provide a proper REPRoducible EXample (reprex) illustrating your issue.