Illustrator / photoshop advice

[JABB]

I have a vector image saved in eps format, which I am told is endlessly resizable. I wish to use to wrap a van at some point.
Now, it was green background with white design. I have changed the background to red, which I want, but I now want to change some of the white to another colour too.
If I do this in photoshop, I get the desired effect by filling with the bucket tool, but the file changes to a sized file. In illustrator, I can't seem to just choose a small part to fill.
Any advice? I can send images to show what I am talking about.
Thanks

[MysteryLemon]

An eps file is basically lines. They are infinitely resizable as they aren't a rendered format. When you load it into photoshop, photoshop rasterises/renders the image so you can work with it. Photoshop isn't really ideal for working with vectors.

Illustrator would be best but whether you can set the colours how you like depends on how the image had been drawn. its likely the whole if the artwork will gave been expanded into one shape rather than loads if individual objects. You will be able to edit the artwork and give the area inside them a fill but to the sign wrighter, this makes no difference. You could set the colours to whatever you like but the signwriter just sees the lines and uses them to plot onto a coloured vinyl. The colour in the file doesn't matter.

Use photoshop to do what you like, print it and give that to the signwriter along with the eps. They will do the rest.