What are these tiny yellow bugs on the vegetable leaves in my greenhouse?

They are very small 0.5 - 2 mm I did not realise it was bug until I looked closely and it moved. There are several on the tops of the leaves on the bottom layers of my tomato plant, also several on the melon plant which I have in the same grow-bag. They are bright lemon yellow and may be suckers not chewers as there are no holes in the leaves just small brown dry patches. Is it Aphids? (im a garden newbie)