Thanks guys for all these replies !
Teknopants : Thanks a lot ! I'm working on some 32x32 tiles as you said, you're helping me quite well with this

You are right about the darker rock tiles, in fact i've juste darkened the basic rock tiles but it doesn't seem to work. Seems I'll have to make a proper set of darker tiles.
Nope, won't use it for any game. Or at least it's not decided yet. However, working on these gave me some game ideas ! So, eventually if I go further enough with these I can use them
Seiseki : Yeah, readability is an issue. I think the way I put tiles together did affect readability too. I mean, if i decide that darker tiles will be background and the lighter, more solid tiles will be foregro)und, I think it could work
I made another mock-up, trying to make it more platformer like.

The red tiles are those you can walk on, the others are part of the background. Not sure it's really clear yet, at least it looks less random (I hope)
Pypebros : Thanks for giving you opinion ! You are right about those rock tiles, I must admit they are pretty uncomfortable to use because some combinations of tiles won't work due to that issue. I might rework them a bit so they combine better and look less flat. Also, you mentionned a glitch effect, It's pretty much like half-intended. I like this buggy style, when it's not to obvious.
Probo : Thank you

. Are you saying that I shouldn't mix dirt and metal tiles ?
r4c7 : Thank you for your attention. I'll be working on the blending issue, but I don't want it to be too smooth. Just like you I do like the grid being visible.
You're right about the palette being a little bland. Will surely tweak the contrasts a bit, not too much thought. I like this kind of monochrome dirty look, makes it cyber-punk and polluted, that's pretty much what I was going for. Not sure about the saturation on the rocks, will give it a try anyways
Wurfle : Yeah, the colors are not completely defenitive. I agree with you for the darker brown, I'm a big fan of hue shiftings but this brown will be the basic color of the darker rock tiles and I don't want them to be too purple. But be sure I'll proceed at some tests about palette
