Egg shell discolouring

I have 6 hybrids & 2 Brahma hens  all approx 24 weeks old .  Only two of the Hybrids have started laying (they did straight away from when I got them  @17wks) The others are not laying yet.   However I have just found an egg in the COOP though not in the nest boxes  that that is the usuall brown but has a pale discolouring(white-ish) over most of it. 
 Any idea what that means ?

There is quite a range of shell colours and patterns.  I have pullets who lay speckled eggs, one lays eggs that always have a slightly rough patch at one end, and sometimes I get eggs with little chalky white bits on the shell.

I think you probably have a new girl who has just started laying and you found her first egg.  Sometimes the first egg can sneak up on them and they just lay it where they are standing.  She may always lay eggs with these colourings, or it may just be that her egg-laying machinery needs to get into gear before she settles into her 'normal' shell pattern.

Also 2 days ago the white-tip (one of the layers) was ever so keen to get out of the pen to the point of trying to force her head through the wire.   When I let them all out  she went off  on her own to a part of my land that I'd never seen them & was sitting on a clutch of 3 eggs. Up untill then she had been laying in the nest box .  since then each morning she has made her way to that spot . I have removed the eggs but how do I stop this . ?

Are the nests you provide nice and comfortable and quiet?  Do you have some fake eggs in the nest boxes?  These would be my first suggestions.

Then go to the nest she has made for herself and thoroughly disturb it.  Remove any straw or comfortable material, maybe put a plant pot in the space - whatever you need to do to make it an undesirable nest.

Then keep Ms White-Tip in her pen until after she has laid - no matter how determined she is to get out!  She needs to get back in the habit of laying in the nest box you have provided.