This will ultimately depend on how each of your projects were / are being printed.
Often, stationery such as letterheads and envelopes are printed in only 1 or two colours, and can be a Pantone® (PMS) colour.
Brochures, which often will include photos, backgrounds and multiple colours for visual effect, are printed “full colour” or CMYK.
This will mean that your logo will most likely look different on each project as different kinds of inks/colours are being used to reproduce your brand.
If you have a logo or brand that you simply must have consistent throughout all of your brochures / stationery, please be sure to mention when contacting us to print your project. This may determine the type of stock, number of inks used, special treatments applied (i.e. gloss coatings etc).