well, 50 is kinda cute, i have more than three times of that amount in space marines...

and you think painting 50 guardsmen is tedious? ask my IG playing buddy who just a few days ago got sentenced to paint his entire infantry by our small player community.
he had about 450 men at the last counting.

well, okay, if you want real advide: do it the way i do: start with an extremely crappy 3-color paintjob. looks ugly and only halfway finished, but when seen as an army on the board, it looks better to have the entire army semi-painted instead of five models with a good paintjob and the rest without any color.
then, you can improve the painting step by step, going through the army.
example given with how our IG man does it ith his cadians:
-paint the armor on all models green
-paint the clothes on all models.
enough for the first part, you can play games with that. better than unpainted.
next big spree:
-paint all boots brown
-paint all gun parts that need to silver
enough for good looks from distance.
further improvement:
-all faces in skin color
-some melta or plasmagun colors, and some lamps, lights, flashlights or such thingys
at this point, the army looks good from normal gaming distance on the board. no display minis, but looks good in games.

(if you are motivated enough, you can add steps for further improvement here, like washes or drybrushing, and so on, depends if you want to do that with a mass army

)