Greenish means a little bit green, not a strong, clear green. The colour is mixed or pale: green mixed with grey, yellow, brown, or just not quite there yet. People say it of water, the sky before a storm, a bruise, a face when someone feels sick, cloth that has faded, or fruit that is not ripe. It is an everyday word, not fancy. Though actually, -ish here is doing quiet work: it pulls the colour back. A green door is green; a greenish door only leans that way. You can do the same with other colours (reddish, bluish), and with time and number in casual talk (sixish, youngish), but that last use is spoken and loose. Greenish itself stays with colour, and sometimes with a slightly unwell look.