The quality of acrylic paint - not as obvious as you might think...

Causes me to reflect that while I've drawn myself, I've never painted a self-portrait.  There are some things that are terrifying in prospect. I must now go to the Gallery, on which Alan advises me he might have been overly harsh about my recent opus.  Heaven knows, one isn't the kind fo chap to bear a grudge or seek petty revenge.  It's just that Alan might need to take extra care should a shadowy figure with a catapult be observed lurking in the shrubbery.  

Actually, he was quite kind; in that Bickley-esque way we know and love... Interested in the "ditch the birds .... or at least group them"; I do add birds, usually in the hope of suggesting some kind of life, which means they're an after-thought; do I agree with Alan .... well, for once, I think I don't; though I do certainly agree about the value of Burnt Sienna as opposed to Burnt Umber: BU is such a mucky colour - but all I had in that paint range which I wanted to test.  My considered advice to you all, therefore: ditch the Burnt Umber - in oil it can just sink in, in acrylic it's maybe OK for ditch-water, in watercolour - well Alan Owen uses it sometimes: but I've never yet got the results with it that he can.  

I did miss it, sorry - Burnt Umber can modify the yellows, that is darken or desaturate them, and it will darken red, though at the cost of making it distinctly murky.  If you want to use it as a darkening agent, mix it with a little Ultramarine (not the other blues, because most of them will turn it a deep green - mind you, that may be very useful sometimes). Generally speaking, the best way to darken a colour is to mix its complementary with it.  So - a red will darken a green; green will darken a red - directly, or optically (in a glaze, that is, for the latter).  Blues come in a range of quite satisfactory darks anyway, so you'll rarely need to darken them, but the best way to do that is just add a deeper blue.  Yellows and oranges can be darkened with violet.  Experiment with colour swatches - not forgetting to make a note of what you've used.   Take a look at the split primary system - well explained by the Michael Wilcox School of Colour - or my e-book, of which I believe you have a copy: it deal with oil paint, but the principles involved are the same, even if many of the colours vary.  The problem with darkening any colours with black or brown (or Payne's Grey, which is very often used) is that they are almost absent of colour - well, black is totally absent of colour.  So what you'll get is often a sooty, ditch-water dark which just sits on the canvas and glowers at you sulkily.  Colour darkens - or lightens - colour better than anything else.   If you want a really dark colour, just a little of which can produce a range of greys (in oil, acrylic, and watercolour) try Burnt Sienna rather than Burnt Umber; it's basically an earth orange, so full of colour, and it makes great darks mixed with Ultramarine.  Other good darks can be made with a dark green, like (if we're talking about acrylic in this case) Pthalo Green, or Viridian if we're talking about oil, with any number of reds - crimsons or Burnt Sienna again probably work best, but a dark green plus even Cadmium Red, or Quinacridone red/violet, will give you intense darks, or, with added white (or water) gentle greys. There are many other darks which don't leave you with a muddy smear rather than the warm dark you were looking for, before you need to reach for the black, Payne's, or Burnt Umber.  All of those CAN give you satisfactory darks, but if you use them, mix a  bit of real colour in with them - a bit of red in black can give it quite a lift, for instance.   PS - the distinction between Pthalo and real Viridian in acrylic - if there's a hue named Viridian in acrylic ranges, it'll be a form of Pthalo Green, because the real pigment isn't compatible with acrylic resin.  If Syd Edward were still around, he'd be jumping up and down at this point and saying they're both horrible colours and you shouldn't use them, but - well, God bless him, but they're very good colours if you just know what you're doing with them and, particularly with the Pthalo, are careful of its tendency to take over your painting.