Le Rouge et le vert, avec un soupçon de bleu
In this challenge, we just have this image, but only the end is interesting:
If we try to decode the text 76 321021089710332 115116581089795118 95 1109599 108 114115125, we get L flag st:la_v_n_clrs}. If we replace the missing values by Xs, we get LX flag Xst:la_vXX_Xn_cXXlXXrs}. So first, we can notice that the first part should be Le flag est. From this we could have guessed that a space is equivalent to an e and so get Le flag est:la_vXe_en_cXXleXrs}. Now we are just missing a few characters. We and guess the i of vie and the word couleur can also be guessed.
This gives us the flag 404CTF{la_vie_en_couleurs}