Your 50c is Scott 699; 683 would have the imprint SANCHEZ TODA below the frame at lower right:
It looks like the engraved issues from about that time can have either grayish white or pale straw (or toned) paper. It might be just age, but I doubt it. These definitives were used for up to about 15 years, so there are several printings of them. Then there's also likely going to be some variation in paper, ink shades, etc. It would take a dedicated specialist to sort that stuff out.
Here's my 2 Scott 695a, two different shades with the caveat that used stamps might have been affected by exposure to sunlight or chemicals:

grayish white

pale straw
your 20c is Scott 693a, perf 13 x 13.5:
I believe that's correct, the lines have something to do with the gumming machinery/gumming process.
My pale straw stamp also has those lines, magnified here:
