There are many carbon patterns, but this one is tiny.
Source Designova
Seamless Green Tile Background
Source V. Hartikainen
Background Wall, Art Abstract, Blue Well & CC0 texture.
Source Ractapopulous
The image depicts meshed silhouettes of various things.The original image is an OCAL clipart called "Enter FOSSASIA 2016 #IoT T-shirt Design Contest" uploaded by "openclipart".Thanks.
Source Yamachem
Small gradient crosses inside 45-degree boxes, or bigger crosses if you will.
Source Wassim
The green fibers pattern will work very well in grayscale as well.
Source Matteo Di Capua
A free seamless background with pink spots.
Source V. Hartikainen
From a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
To get the tile this is based on select the rectangle in Inkscape and use shift-alt-i
Source Firkin
A monochrome pattern from a tile that can be had by selecting the rectangle in Inkscaope and using shift+alt+i
Source Firkin
Prismatic Curved Diamond Pattern 2 No Background
Source GDJ
To get the tile this is based on select the rectangle in Inkscape and use shift+alt+i
Source Firkin
Colourful background achieved with gradient fills.
Source Firkin
The image is a design of blue glass.How about using it as background image?
Source Yamachem
High detail stone wall with minor cracks and specks.
Source Projecteightyfive
Cubes as far as your eyes can see. You know, because they tile.
Source Jan Meeus
ZeroCC tileable mossy (lichen) stone texture, edited from pixabay. CC0
Source Sojan Janso
From a drawing in 'Worsborough; its historical associations and rural attractions', Joseph Wilkinson, 1879.
Source Firkin
From a drawing in 'Chambéry à la fin du XIVe siècle', Timoleon Chapperon, 1863.
Source Firkin
Bit of a strange name on this one, but still nice. Tiny gray square things.
Source Carlos Valdez