There are lots of ways to approach something like this. I'd go about it this way, which:
- quickly gives you a field of triangles (or other tessellating shapes) to get creative on, with minimal dull repetitive work
- can't introduce any imperfections or irregularities that would come back and hit you
(these steps are for Illustrator: just realised I hadn't said that explicitly. It would be a massive pain to do this in Photoshop. But it seemed like the asker had already figured that out)
Create a square of triangles:
1: Create the base shape of the pattern. In this case, it's a square.
2: With the Smart Guides green snapping guide lines (
cmd-u) turned on, draw lines precisely connecting opposite corners
Divide in the Pathfinder window to chop the square into 4 identical triangles
Make a precise grid from them: (you could also use Patterns for this, then expand the pattern, which would work better if it was a less regular shape)
alt and tap up a number of times to create that many identical copies
5: In the
Align window, switch to
Align to key object then use distribute space with 0px in the box to create an even column with no gaps
6: Same thing going the other way. Ungroup everything, remove any strokes. You've now got a field of triangles to get creative with - from here on, it's just selecting triangles and experimenting with colour, and there's no more boring drudge work.